doc: add documentation for buffer.byteOffset

Also document a common issue when casting a Buffer object to a
TypedArray object.

Fixes: https://github.com/nodejs/node/issues/19301

PR-URL: https://github.com/nodejs/node/pull/21718


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>