mime: fix docs for WordDecoder.Decode

It was correct for an early version of the CL which introduced the
type, but later versions of the CL changed the behavior without
updating the documentation.

Fixes #12568

Change-Id: Ia4090a02ba122e9f8317ed86c4c9839ae2c539e0
Reviewed-on: https://go-review.googlesource.com/14496Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/mime/encodedword.go

@@ -98,8 +98,7 @@ type WordDecoder struct {
 	CharsetReader func(charset string, input io.Reader) (io.Reader, error)
 }
 
-// Decode decodes an encoded-word. If word is not a valid RFC 2047 encoded-word,
-// word is returned unchanged.
+// Decode decodes an RFC 2047 encoded-word.
 func (d *WordDecoder) Decode(word string) (string, error) {
 	fields := strings.Split(word, "?") // TODO: remove allocation?
 	if len(fields) != 5 || fields[0] != "=" || fields[4] != "=" || len(fields[2]) != 1 {