documentation: clarify notes for clean.requireForce

Add "-i" (interactive clean option) to clarify the documentation for
"clean.requireForce" config variable.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jiangxin

Documentation/config.txt

@@ -795,8 +795,8 @@ browser.<tool>.path::
 	working repository in gitweb (see linkgit:git-instaweb[1]).
 
 clean.requireForce::
-	A boolean to make git-clean do nothing unless given -f
-	or -n.   Defaults to true.
+	A boolean to make git-clean do nothing unless given -f,
+	-i or -n.   Defaults to true.
 
 color.branch::
 	A boolean to enable/disable color in the output of