doc: reword message.headers to indicate they are not read-only

message.headers states that the headers are read-only, when in fact they
are not. This change rewords the docs to a more appropriate description,
while not promoting this kind of behavior.

Fixes: #3146
PR-URL: https://github.com/nodejs/node/pull/3814


Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>