git-send-email: Document support for local sendmail instead of SMTP server

Fix the --smtp-server option description to match reality.
Signed-off-by: NSergey Vlasov <vsu@altlinux.ru>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-send-email.txt

@@ -66,8 +66,11 @@ The options available are:
 	all that is output.
 
 --smtp-server::
-	If set, specifies the outgoing SMTP server to use.  Defaults to
-	localhost.
+	If set, specifies the outgoing SMTP server to use.  A full
+	pathname of a sendmail-like program can be specified instead;
+	the program must support the `-i` option.  Defaults to
+	`/usr/sbin/sendmail` or `/usr/lib/sendmail` if such program is
+	available, or to `localhost` otherwise.
 
 --subject::
    	Specify the initial subject of the email thread.