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>