CMP app and doc: improve texts on (un-)trusted certs, srvCert, etc.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/20277)

(cherry picked from commit d5e50bdf)