doc: deprecate buffer's parent property

`buffer.parent` property is actually an alias of `buffer.buffer`
property. This patch actually doc-deprecates it and points the users to
the `buffer.buffer` property.

PR-URL: https://github.com/nodejs/node/pull/8332


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>