errors: improve godoc for Join and Unwrap

Clarify how to deconstruct the error returned by errors.Join, and how Unwrap interacts
with errors.Join.

Author: dnephin

Diff for: src/errors/join.go

@@ -10,6 +10,8 @@ package errors
 // The error formats as the concatenation of the strings obtained
 // by calling the Error method of each element of errs, with a newline
 // between each string.
+//
+// A non-nil error returned by Join implements the Unwrap() []error method.
 func Join(errs ...error) error {
 	n := 0
 	for _, err := range errs {

Diff for: src/errors/wrap.go

@@ -12,7 +12,7 @@ import (
 // type contains an Unwrap method returning error.
 // Otherwise, Unwrap returns nil.
 //
-// Unwrap returns nil if the Unwrap method returns []error.
+// Unwrap returns nil if the signature of err.Unwrap returns []error.
 func Unwrap(err error) error {
 	u, ok := err.(interface {
 		Unwrap() error