Add detailed comments and return descriptions to Part methods

Enhanced the documentation of `Part` struct and its methods to provide clearer explanations, including parameter and return descriptions. This improves code readability and helps developers understand the functionality and usage of each method.

part.go

@@ -12,7 +12,11 @@ import (
 // PartOption returns a function that can be used for grouping Part options
 type PartOption func(*Part)
 
-// Part is a part of the Msg
+// Part is a part of the Msg.
+//
+// This struct represents a single part of a multipart message. Each part has a content type,
+// charset, optional description, encoding, and a function to write its content to an io.Writer.
+// It also includes a flag to mark the part as deleted.
 type Part struct {
  contentType ContentType
  charset Charset
@@ -22,7 +26,14 @@ type Part struct {
  writeFunc func(io.Writer) (int64, error)
 }
 
-// GetContent executes the WriteFunc of the Part and returns the content as byte slice
+// GetContent executes the WriteFunc of the Part and returns the content as a byte slice.
+//
+// This function runs the part's writeFunc to write its content into a buffer and then returns
+// the content as a byte slice. If an error occurs during the writing process, it is returned.
+//
+// Returns:
+// - A byte slice containing the part's content.
+// - An error if the writeFunc encounters an issue.
 func (p *Part) GetContent() ([]byte, error) {
  var b bytes.Buffer
  if _, err := p.writeFunc(&b); err != nil {
@@ -31,83 +42,172 @@ func (p *Part) GetContent() ([]byte, error) {
  return b.Bytes(), nil
 }
 
-// GetCharset returns the currently set Charset of the Part
+// GetCharset returns the currently set Charset of the Part.
+//
+// This function returns the Charset that is currently set for the Part.
+//
+// Returns:
+// - The Charset of the Part.
 func (p *Part) GetCharset() Charset {
  return p.charset
 }
 
-// GetContentType returns the currently set ContentType of the Part
+// GetContentType returns the currently set ContentType of the Part.
+//
+// This function returns the ContentType that is currently set for the Part.
+//
+// Returns:
+// - The ContentType of the Part.
 func (p *Part) GetContentType() ContentType {
  return p.contentType
 }
 
-// GetEncoding returns the currently set Encoding of the Part
+// GetEncoding returns the currently set Encoding of the Part.
+//
+// This function returns the Encoding that is currently set for the Part.
+//
+// Returns:
+// - The Encoding of the Part.
 func (p *Part) GetEncoding() Encoding {
  return p.encoding
 }
 
-// GetWriteFunc returns the currently set WriterFunc of the Part
+// GetWriteFunc returns the currently set WriteFunc of the Part.
+//
+// This function returns the WriteFunc that is currently set for the Part, which writes
+// the part's content to an io.Writer.
+//
+// Returns:
+// - The WriteFunc of the Part, which is a function that takes an io.Writer and returns
+// the number of bytes written and an error (if any).
 func (p *Part) GetWriteFunc() func(io.Writer) (int64, error) {
  return p.writeFunc
 }
 
-// GetDescription returns the currently set Content-Description of the Part
+// GetDescription returns the currently set Content-Description of the Part.
+//
+// This function returns the Content-Description that is currently set for the Part.
+//
+// Returns:
+// - The Content-Description of the Part as a string.
 func (p *Part) GetDescription() string {
  return p.description
 }
 
-// SetContent overrides the content of the Part with the given string
+// SetContent overrides the content of the Part with the given string.
+//
+// This function sets the content of the Part by creating a new writeFunc that writes the
+// provided string content to an io.Writer.
+//
+// Parameters:
+// - content: The string that will replace the current content of the Part.
 func (p *Part) SetContent(content string) {
  buffer := bytes.NewBufferString(content)
  p.writeFunc = writeFuncFromBuffer(buffer)
 }
 
-// SetContentType overrides the ContentType of the Part
+// SetContentType overrides the ContentType of the Part.
+//
+// This function sets a new ContentType for the Part, replacing the existing one.
+//
+// Parameters:
+// - contentType: The new ContentType to be set for the Part.
 func (p *Part) SetContentType(contentType ContentType) {
  p.contentType = contentType
 }
 
-// SetCharset overrides the Charset of the Part
+// SetCharset overrides the Charset of the Part.
+//
+// This function sets a new Charset for the Part, replacing the existing one.
+//
+// Parameters:
+// - charset: The new Charset to be set for the Part.
 func (p *Part) SetCharset(charset Charset) {
  p.charset = charset
 }
 
-// SetEncoding creates a new mime.WordEncoder based on the encoding setting of the message
+// SetEncoding creates a new mime.WordEncoder based on the encoding setting of the message.
+//
+// This function sets a new Encoding for the Part, replacing the existing one.
+//
+// Parameters:
+// - encoding: The new Encoding to be set for the Part.
 func (p *Part) SetEncoding(encoding Encoding) {
  p.encoding = encoding
 }
 
-// SetDescription overrides the Content-Description of the Part
+// SetDescription overrides the Content-Description of the Part.
+//
+// This function sets a new Content-Description for the Part, replacing the existing one.
+//
+// Parameters:
+// - description: The new Content-Description to be set for the Part.
 func (p *Part) SetDescription(description string) {
  p.description = description
 }
 
-// SetWriteFunc overrides the WriteFunc of the Part
+// SetWriteFunc overrides the WriteFunc of the Part.
+//
+// This function sets a new WriteFunc for the Part, replacing the existing one. The WriteFunc
+// is responsible for writing the Part's content to an io.Writer.
+//
+// Parameters:
+// - writeFunc: A function that writes the Part's content to an io.Writer and returns
+// the number of bytes written and an error (if any).
 func (p *Part) SetWriteFunc(writeFunc func(io.Writer) (int64, error)) {
  p.writeFunc = writeFunc
 }
 
-// Delete removes the current part from the parts list of the Msg by setting the
-// isDeleted flag to true. The msgWriter will skip it then
+// Delete removes the current part from the parts list of the Msg by setting the isDeleted flag to true.
+//
+// This function marks the Part as deleted by setting the isDeleted flag to true. The msgWriter
+// will skip over this Part during processing.
 func (p *Part) Delete() {
  p.isDeleted = true
 }
 
-// WithPartCharset overrides the default Part charset
+// WithPartCharset overrides the default Part charset.
+//
+// This function returns a PartOption that allows the charset of a Part to be overridden
+// with the specified Charset.
+//
+// Parameters:
+// - charset: The Charset to be set for the Part.
+//
+// Returns:
+// - A PartOption function that sets the Part's charset.
 func WithPartCharset(charset Charset) PartOption {
  return func(p *Part) {
  p.charset = charset
  }
 }
 
-// WithPartEncoding overrides the default Part encoding
+// WithPartEncoding overrides the default Part encoding.
+//
+// This function returns a PartOption that allows the encoding of a Part to be overridden
+// with the specified Encoding.
+//
+// Parameters:
+// - encoding: The Encoding to be set for the Part.
+//
+// Returns:
+// - A PartOption function that sets the Part's encoding.
 func WithPartEncoding(encoding Encoding) PartOption {
  return func(p *Part) {
  p.encoding = encoding
  }
 }
 
-// WithPartContentDescription overrides the default Part Content-Description
+// WithPartContentDescription overrides the default Part Content-Description.
+//
+// This function returns a PartOption that allows the Content-Description of a Part
+// to be overridden with the specified description.
+//
+// Parameters:
+// - description: The Content-Description to be set for the Part.
+//
+// Returns:
+// - A PartOption function that sets the Part's Content-Description.
 func WithPartContentDescription(description string) PartOption {
  return func(p *Part) {
  p.description = description