doc: Adding best practises for crypto.pbkdf2

Added some information around usages of how to use iterations, how to
choose decent salts and updating the example to have a significant
work factor and to use sha512.

PR-URL: https://github.com/nodejs/node/pull/3290


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>