doc: add `added:` information for buffer

Add `added:` and `deprecated:` entries to buffer.md.
These are incomplete (particularly for some of the ancient features),
but correct to the best of my knowledge. This serves as a
demonstration of how the `added:`/`deprecated:` metadata may be
implemented in 'real' docs.

PR-URL: https://github.com/nodejs/node/pull/6495


Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>