s_server: make -rev option easier to find (mention echo)

s_server: make -rev option easier to find (mention echo)

Since the service is echo-like (see TCP port 7 from RFC 862 or
gnutls-serv --echo), make it easier to find by mentioning "echo" in
the description of it in the help message an man page

Also fixes the man page inconsistency ("sends it back to the server")

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/15739)