Make SNI behavior more clear in s_client doc & help

Update s_client -help and pod file.

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3654)