encoding/json: fix docs on valid key names

This has been inaccurate since https://golang.org/cl/6048047.

Fixes #15317.

Change-Id: If93d2161f51ccb91912cb94a35318cf33f4d526a
Reviewed-on: https://go-review.googlesource.com/23691Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/encode.go

@@ -90,8 +90,8 @@ import (
 //    Int64String int64 `json:",string"`
 //
 // The key name will be used if it's a non-empty string consisting of
-// only Unicode letters, digits, dollar signs, percent signs, hyphens,
-// underscores and slashes.
+// only Unicode letters, digits, and ASCII punctuation except quotation
+// marks, backslash, and comma.
 //
 // Anonymous struct fields are usually marshaled as if their inner exported fields
 // were fields in the outer struct, subject to the usual Go visibility rules amended