Documentation/submodule: Add --force to update synopsis

In commit 9db31bd (submodule: Add --force option for git submodule
update, 2011-04-01) we added the option to the implementation's usage
synopsis but forgot to add it to the synopsis in the command
documentation.  Add the option to the synopsis in the same location it
is reported in usage and re-wrap the options to avoid long lines.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 	      [--reference <repository>] [--] <repository> [<path>]
 'git submodule' [--quiet] status [--cached] [--recursive] [--] [<path>...]
 'git submodule' [--quiet] init [--] [<path>...]
-'git submodule' [--quiet] update [--init] [-N|--no-fetch] [--rebase]
+'git submodule' [--quiet] update [--init] [-N|--no-fetch] [-f|--force] [--rebase]
 	      [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
 'git submodule' [--quiet] summary [--cached|--files] [(-n|--summary-limit) <n>]
 	      [commit] [--] [<path>...]