net/http: add godoc for Dir.Open function

This commit adds godoc for Dir.Open function.

Change-Id: Ibc3b22f38660a082802e1f868c5cf9d880fc2801
GitHub-Last-Rev: 774cfd7d8cc61989179956e47d51451135b6c203
GitHub-Pull-Request: golang/go#30353
Reviewed-on: https://go-review.googlesource.com/c/163437Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/fs.go

@@ -63,6 +63,8 @@ func mapDirOpenError(originalErr error, name string) error {
 	return originalErr
 }
 
+// Open implements FileSystem using os.Open, opening files for reading rooted
+// and relative to the directory d.
 func (d Dir) Open(name string) (File, error) {
 	if filepath.Separator != '/' && strings.ContainsRune(name, filepath.Separator) {
 		return nil, errors.New("http: invalid character in file path")