submodule: Document the details of the command line syntax

Only "status" accepts "--cached" and the preferred way of
passing sub-command specific options is after the sub-command.

The documentation is adapted to reflect this.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -9,8 +9,9 @@ git-submodule - Initialize, update or inspect submodules
 SYNOPSIS
 --------
 [verse]
-'git-submodule' [--quiet] [-b branch] add <repository> [<path>]
-'git-submodule' [--quiet] [--cached] [status|init|update] [--] [<path>...]
+'git-submodule' [--quiet] add [-b branch] [--] <repository> [<path>]
+'git-submodule' [--quiet] status [--cached] [--] [<path>...]
+'git-submodule' [--quiet] [init|update] [--] [<path>...]
 
 
 COMMANDS