doc: add return values in crypto documentation

Clarify return values for crypto.publicEncrypt and similar functions

PR-URL: https://github.com/nodejs/node/pull/16229
Fixes: https://github.com/nodejs/node/issues/12946


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>