doc: sort buffer alphabetically

Reorders, with minimal contextual duplication, the buffer documentation
alphabetically.

PR-URL: https://github.com/nodejs/node/pull/3662


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>