doc: warn about using strings as inputs in crypto

PR-URL: https://github.com/nodejs/node/pull/37248


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>