openssl-pkeyutl.pod.in: add that -sign is default op, update claim on hash needed...

openssl-pkeyutl.pod.in: add that -sign is default op, update claim on hash needed for -sign/-verify, etc.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/25913)

(cherry picked from commit ded80b3cc7b36f8ce425e4d53691920dfa875451)
(cherry picked from commit 9eeaac96464b7adfef5b6bc77dc4bfda12f065e0)