config.txt: move clean.* to a separate file

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -307,9 +307,7 @@ include::config/browser.txt[]
 
 include::config/checkout.txt[]
 
-clean.requireForce::
-	A boolean to make git-clean do nothing unless given -f,
-	-i or -n.   Defaults to true.
+include::config/clean.txt[]
 
 color.advice::
 	A boolean to enable/disable color in hints (e.g. when a push

Documentation/config/clean.txt

@@ -0,0 +1,3 @@
+clean.requireForce::
+	A boolean to make git-clean do nothing unless given -f,
+	-i or -n.   Defaults to true.