doc: document that whitespace is ignored in base64 decoding

Fixes: https://github.com/nodejs/node/issues/8569

PR-URL: https://github.com/nodejs/node/pull/34227


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>