gitmodules: clarify the ignore option values

Add more structure and describe each possible option in a self-contained
way, not referring to any of the previously described options.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitmodules.txt

@@ -66,17 +66,26 @@ submodule.<name>.fetchRecurseSubmodules::
 
 submodule.<name>.ignore::
 	Defines under what circumstances "git status" and the diff family show
-	a submodule as modified. When set to "all", it will never be considered
+	a submodule as modified. The following values are supported:
-	modified (but will nonetheless show up in the output of status and
+
-	commit when it has been staged), "dirty" will ignore all changes
+	all;; The submodule will never be considered modified (but will
-	to the submodules work tree and
+	    nonetheless show up in the output of status and commit when it has
-	takes only differences between the HEAD of the submodule and the commit
+	    been staged).
-	recorded in the superproject into account. "untracked" will additionally
+
-	let submodules with modified tracked files in their work tree show up.
+	dirty;; All changes to the submodule's work tree will be ignored, only
-	Using "none" (the default when this option is not set) also shows
+	    committed differences between the HEAD of the submodule and its
-	submodules that have untracked files in their work tree as changed.
+	    recorded state in the superproject are taken into account.
-	If this option is also present in the submodules entry in .git/config of
+
-	the superproject, the setting there will override the one found in
+	untracked;; Only untracked files in submodules will be ignored.
+	    Committed differences and modifications to tracked files will show
+	    up.
+
+	none;; No modifiations to submodules are ignored, all of committed
+	    differences, and modifications to tracked and untracked files are
+	    shown. This is the default option.
+
+	If this option is also present in the submodules entry in .git/config
+	of the superproject, the setting there will override the one found in
 	.gitmodules.
 	Both settings can be overridden on the command line by using the
 	"--ignore-submodule" option. The 'git submodule' commands are not