Documentation/git-clean.txt: document that -f may need to be given twice

This is needed in build automation where the tree really needs to
be reset to known state.
Signed-off-by: NMikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -34,8 +34,12 @@ OPTIONS
 -f::
 --force::
 	If the Git configuration variable clean.requireForce is not set
-	to false, 'git clean' will refuse to run unless given -f, -n or
-	-i.
+	to false, 'git clean' will refuse to delete files or directories
+	unless given -f, -n or -i. Git will refuse to delete directories
+	with .git sub directory or file unless a second -f
+	is given. This affects also git submodules where the storage area
+	of the removed submodule under .git/modules/ is not removed until
+	-f is given twice.
 
 -i::
 --interactive::