doc: clarify memory sharing behavior of buffer ctor

* Clarify that memory is always shared and never copied.
* Fix wording that sounded like ArrayBuffer has a buffer property.

PR-URL: https://github.com/nodejs/node/pull/10778
Ref: https://github.com/nodejs/node/issues/10770


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>