doc: more explicit crypto.pseudoRandomBytes docs

Updates the docs for the crypto.pseudoRandomBytes function
to more explicitly detail how it's the same as crypto.randomBytes
just without a safety net (e.g. it doesn't throw an error when
there is low entropy).

PR-URL: https://github.com/iojs/io.js/pull/545


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>