Documentation/git-send-pack.txt: document --signed

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-pack.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git send-pack' [--all] [--dry-run] [--force] [--receive-pack=<git-receive-pack>]
-		[--verbose] [--thin] [--atomic]
+		[--verbose] [--thin] [--atomic] [--signed]
 		[<host>:]<directory> [<ref>...]
 
 DESCRIPTION
@@ -69,6 +69,14 @@ be in a separate packet, and the list must end with a flush packet.
 	fails to update then the entire push will fail without changing any
 	refs.
 
+--signed::
+	GPG-sign the push request to update refs on the receiving
+	side, to allow it to be checked by the hooks and/or be
+	logged.  See linkgit:git-receive-pack[1] for the details
+	on the receiving end.  If the attempt to sign with `gpg` fails,
+	or if the server does not support signed pushes, the push will
+	fail.
+
 <host>::
 	A remote host to house the repository.  When this
 	part is specified, 'git-receive-pack' is invoked via