doc: document PKCS12 password prompting for certificates

doc: document PKCS12 password prompting for certificates

Document that commands reading certificates from PKCS#12 files may
prompt for a password. The existing documentation only mentioned
password prompting for private keys.

Fixes #21292

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
MergeDate: Fri Apr  3 15:52:28 2026
(Merged from https://github.com/openssl/openssl/pull/29918)

(cherry picked from commit c7b283263290e5bbcb3aa64af5e983ae73106268)