doc: document base64url encoding support

Documents that creating a buffer from base64 encoded string
transparently supports "URL and Filename Safe Alphabet" base64url
encoding.

PR-URL: https://github.com/nodejs/node/pull/5243


Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: Benjamin Gruenbaum <inglor@gmail.com>
Reviewed-By: Alejandro Oviedo <alejandro.oviedo.g@gmail.com>