doc: explain `hex` encoding in Buffer API

fixes: https://github.com/nodejs/node/issues/29786
refs: https://github.com/nodejs/node/pull/29792
refs: https://github.com/nodejs/node/issues/24491

PR-URL: https://github.com/nodejs/node/pull/31352
Fixes: https://github.com/nodejs/node/issues/29786
Refs: https://github.com/nodejs/node/pull/29792
Refs: https://github.com/nodejs/node/issues/24491


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>