golint: add missing GoDoc

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

types.go

@@ -32,7 +32,16 @@ var (
 	registry = make(map[reflect.Type]string)
 )
 
-var ErrNotFound = errors.New("not found")
+// Definitions of common error types used throughout typeurl.
+//
+// These error types are used with errors.Wrap and errors.Wrapf to add context
+// to an error.
+//
+// To detect an error class, use errors.Is() functions to tell whether an
+// error is of this type.
+var (
+	ErrNotFound = errors.New("not found")
+)
 
 // Register a type with a base URL for JSON marshaling. When the MarshalAny and
 // UnmarshalAny functions are called they will treat the Any type value as JSON.
@@ -119,14 +128,21 @@ func UnmarshalAny(any *types.Any) (interface{}, error) {
 	return UnmarshalByTypeURL(any.TypeUrl, any.Value)
 }
 
+// UnmarshalByTypeURL unmarshals the given type and value to into a concrete type.
 func UnmarshalByTypeURL(typeURL string, value []byte) (interface{}, error) {
 	return unmarshal(typeURL, value, nil)
 }
 
+// UnmarshalTo unmarshals the any type into a concrete type passed in the out
+// argument. It is identical to UnmarshalAny, but lets clients provide a
+// destination type through the out argument.
 func UnmarshalTo(any *types.Any, out interface{}) error {
 	return UnmarshalToByTypeURL(any.TypeUrl, any.Value, out)
 }
 
+// UnmarshalTo unmarshals the given type and value into a concrete type passed
+// in the out argument. It is identical to UnmarshalByTypeURL, but lets clients
+// provide a destination type through the out argument.
 func UnmarshalToByTypeURL(typeURL string, value []byte, out interface{}) error {
 	_, err := unmarshal(typeURL, value, out)
 	return err