send-email: document the --smtp-debug option

The option was already shown in -h output, so it should be documented
in the man page.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Suggested-by: David A. Greene <greened@obbligato.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -198,6 +198,10 @@ must be used for each option.
 	if a username is not specified (with '--smtp-user' or 'sendemail.smtpuser'),
 	then authentication is not attempted.
 
+--smtp-debug=0|1::
+	Enable (1) or disable (0) debug output. If enabled, SMTP
+	commands and replies will be printed. Useful to debug TLS
+	connection and authentication problems.
 
 Automating
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