doc: document use of buffer.swap16() for utf16be

Fixes: https://github.com/nodejs/node/issues/12813

PR-URL: https://github.com/nodejs/node/pull/23747


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>