checkout: mention '--' in the docs

'git checkout' uses '--' to separate options from paths, but it was not
mentioned in the documentation

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git checkout' [-q] [-f] [[--track | --no-track] -b <new_branch> [-l]] [-m] [<branch>]
-'git checkout' [<tree-ish>] <paths>...
+'git checkout' [<tree-ish>] [--] <paths>...
 
 DESCRIPTION
 -----------