Documentation: clarify how to disable elements in core.whitespace

Noticed by Peter Valdemar Mørch.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -358,7 +358,8 @@ core.whitespace::
 	A comma separated list of common whitespace problems to
 	notice.  'git-diff' will use `color.diff.whitespace` to
 	highlight them, and 'git-apply --whitespace=error' will
-	consider them as errors:
+	consider them as errors.  You can prefix `-` to disable
+	any of them (e.g. `-trailing-space`):
 +
 * `trailing-space` treats trailing whitespaces at the end of the line
   as an error (enabled by default).