doc: message.header duplication correction

The old version of the text here was slightly incorrect, so it
just defers the details to the section in which they're fully
described.

PR-URL: https://github.com/nodejs/node/pull/3997


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>