doc: warn that tls.connect() doesn't set SNI

Add a note warning users that when using tls.connect(),
the `servername` option must be set explicitely to enable
SNI, otherwise the connection could fail.

PR-URL: https://github.com/nodejs/node/pull/33855
Fixes: https://github.com/nodejs/node/issues/28167


Co-authored-by: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>