doc: add missing DH_check_pub_key documentation and fix typo

doc: add missing DH_check_pub_key documentation and fix typo

Add DH_check_pub_key to NAME and SYNOPSIS sections. Fix typo where
DH_check_params() was incorrectly written instead of DH_check_params_ex()
in the description of the _ex functions.

Also remove DH_check_pub_key from util/missingcrypto.txt since it is
now documented.

Fixes #8473

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/29768)