Document send-email --smtp-domain

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -119,6 +119,12 @@ Sending
 	value reverts to plain SMTP.  Default is the value of
 	'sendemail.smtpencryption'.
 
+--smtp-domain=<FQDN>::
+	Specifies the Fully Qualified Domain Name (FQDN) used in the
+	HELO/EHLO command to the SMTP server.  Some servers require the
+	FQDN to match your IP address.  If not set, git send-email attempts
+	to determine your FQDN automatically.
+
 --smtp-pass[=<password>]::
 	Password for SMTP-AUTH. The argument is optional: If no
 	argument is specified, then the empty string is used as