encoding/json: add comment for mashalererror struct

Change-Id: Iaabbfe5a4c1bbedd19d4087f1b79e5a38bdd3878
GitHub-Last-Rev: 55c91fc19074dacc66623aa7ff2286b11ccd5340
GitHub-Pull-Request: golang/go#29752
Reviewed-on: https://go-review.googlesource.com/c/157958Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/json/encode.go

@@ -259,6 +259,7 @@ func (e *InvalidUTF8Error) Error() string {
 	return "json: invalid UTF-8 in string: " + strconv.Quote(e.S)
 }
 
+// A MarshalerError represents an error from calling a MarshalJSON or MarshalText method.
 type MarshalerError struct {
 	Type reflect.Type
 	Err  error