githooks documentation: post-checkout hook is also called after clone

The documentation of the post-checkout hook just talks
about git-checkout. But recently git-clone was changed to
call it too, unless the -no-checkout (-n) option is used.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -151,6 +151,10 @@ indicating whether the checkout was a branch checkout (changing branches,
 flag=1) or a file checkout (retrieving a file from the index, flag=0).
 This hook cannot affect the outcome of 'git-checkout'.
 
+It is also run after 'git-clone', unless the --no-checkout (-n) option is
+used. The first parameter given to the hook is the null-ref, the second the
+ref of the new HEAD and the flag is always 1.
+
 This hook can be used to perform repository validity checks, auto-display
 differences from the previous HEAD if different, or set working dir metadata
 properties.