encoding/json: document angle bracket escaping.

Fixes #2643.

R=rsc, d_smithson
CC=golang-dev
https://golang.org/cl/5543068

src/pkg/encoding/json/encode.go

@@ -39,6 +39,8 @@ import (
 //
 // String values encode as JSON strings, with each invalid UTF-8 sequence
 // replaced by the encoding of the Unicode replacement character U+FFFD.
+// The angle brackets "<" and ">" are escaped to "\u003c" and "\u003e"
+// to keep some browsers from misinterpreting JSON output as HTML.
 //
 // Array and slice values encode as JSON arrays, except that
 // []byte encodes as a base64-encoded string.