Add note for non-interactive use of `s_client`

Fixes #8018

Documented the potential issue of premature connection closure in
non-interactive environments, such as cron jobs, when using `s_client`.

Added guidance on using the `-ign_eof` option and input redirection to
ensure proper handling of `stdin` and completion of TLS session data exchange.

Highlight potential issues with the `-ign_eof` flag and provide solutions for
graceful disconnection in SMTP and HTTP/1.1 scenarios to avoid indefinite hangs.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25311)