doc: public keys don't accept passphrases

Since `crypto.publicDecrypt()` and `crypto.publicEncrypt()` accept both
public and private keys, make it clear that the `passphrase` option only
applies to private keys.

PR-URL: https://github.com/nodejs/node/pull/16087
Ref: https://github.com/nodejs/node/pull/16038


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>