encoding/json: Fixed the comment specifying Marshal behavior for maps.

encoding/json: Fixed the comment specifying Marshal behavior for maps.

The comment previously was reversed in sense (it appeared to be
describing unmarshaling). I've fixed that, and added the caveat that map
keys are subject to UTF-8 coercion like other strings.

Change-Id: Id08082aa71401a6e7530a42f979fbb50bd1f4e6a
Reviewed-on: https://go-review.googlesource.com/5221
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>