doc: show keylen in pbkdf2 as a byte length

Ensure that keylen for pbkdf2 is documented as a length of bytes and not
bits.

PR-URL: https://github.com/nodejs/node/pull/3334


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>