doc: add note about key derivation

adds a note to the crypto docs passing along
the advice that openssl gives about what
key derivation function they recommend.

PR-URL: https://github.com/joyent/node/pull/8580


Reviewed-by: Fedor Indutny <fedor@indutny.com>

Cherry-picked-from: https://github.com/joyent/node/commit/7dbc024c8573670170d4f561c01ae892d06399db