Document --ignore-if-in-upstream in git-format-patch

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-format-patch.txt

@@ -12,6 +12,7 @@ SYNOPSIS
 'git-format-patch' [-n | -k] [-o <dir> | --stdout] [--attach] [--thread]
 	           [-s | --signoff] [--diff-options] [--start-number <n>]
 		   [--in-reply-to=Message-Id] [--suffix=.<sfx>]
+		   [--ignore-if-in-upstream]
 		   <since>[..<until>]
 
 DESCRIPTION
@@ -80,6 +81,13 @@ OPTIONS
 	reply to the given Message-Id, which avoids breaking threads to
 	provide a new patch series.
 
+--ignore-if-in-upstream::
+	Do not include a patch that matches a commit in
+	<until>..<since>.  This will examine all patches reachable
+	from <since> but not from <until> and compare them with the
+	patches being generated, and any patch that matches is
+	ignored.
+
 --suffix=.<sfx>::
 	Instead of using `.patch` as the suffix for generated
 	filenames, use specifed suffix.  A common alternative is