submodule update: consistently document the '--checkout' option

Commit 322bb6e12f (add update 'none' flag to disable update of submodule
by default) added the '--checkout' option to "git submodule update" but
forgot to explicitly document it in synopsis, usage string and man page
(It is only mentioned implicitly in the man page). In 23d25e48 (submodule:
explicit local branch creation in module_clone) the synopsis of the man
page was updated, but the "OPTIONS" section of the man page and the usage
string of the git-submodule script still do not mention the '--checkout'
option.

Fix that by documenting this option in usage string and the "OPTIONS"
section of man page too. While at it group the update-mode options into
a single set in the usage string.

Reported-by: Matthijs Kooijman <matthijs@stdin.nl>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -15,7 +15,7 @@ SYNOPSIS
 'git submodule' [--quiet] init [--] [<path>...]
 'git submodule' [--quiet] deinit [-f|--force] [--] <path>...
 'git submodule' [--quiet] update [--init] [--remote] [-N|--no-fetch]
-	      [-f|--force] [--rebase|--merge|--checkout] [--reference <repository>]
+	      [-f|--force] [--checkout|--rebase|--merge] [--reference <repository>]
 	      [--depth <depth>] [--recursive] [--] [<path>...]
 'git submodule' [--quiet] summary [--cached|--files] [(-n|--summary-limit) <n>]
 	      [commit] [--] [<path>...]
@@ -305,6 +305,15 @@ SHA-1.  If you don't want to fetch, you should use `submodule update
 	This option is only valid for the update command.
 	Don't fetch new objects from the remote site.
 
+--checkout::
+	This option is only valid for the update command.
+	Checkout the commit recorded in the superproject on a detached HEAD
+	in the submodule. This is the default behavior, the main use of
+	this option is to override `submodule.$name.update` when set to
+	`merge`, `rebase` or `none`.
+	If the key `submodule.$name.update` is either not explicitly set or
+	set to `checkout`, this option is implicit.
+
 --merge::
 	This option is only valid for the update command.
 	Merge the commit recorded in the superproject into the current branch

git-submodule.sh

@@ -9,7 +9,7 @@ USAGE="[--quiet] add [-b <branch>] [-f|--force] [--name <name>] [--reference <re
    or: $dashless [--quiet] status [--cached] [--recursive] [--] [<path>...]
    or: $dashless [--quiet] init [--] [<path>...]
    or: $dashless [--quiet] deinit [-f|--force] [--] <path>...
-   or: $dashless [--quiet] update [--init] [--remote] [-N|--no-fetch] [-f|--force] [--rebase] [--reference <repository>] [--merge] [--recursive] [--] [<path>...]
+   or: $dashless [--quiet] update [--init] [--remote] [-N|--no-fetch] [-f|--force] [--checkout|--merge|--rebase] [--reference <repository>] [--recursive] [--] [<path>...]
    or: $dashless [--quiet] summary [--cached|--files] [--summary-limit <n>] [commit] [--] [<path>...]
    or: $dashless [--quiet] foreach [--recursive] <command>
    or: $dashless [--quiet] sync [--recursive] [--] [<path>...]"