githooks.txt: clarify push hooks are always executed in $GIT_DIR

Listing the specific hooks might feel verbose but without it the
reader is left to wonder which hooks are triggered during the
push. Something which is not immediately obvious when only trying
to find out where the hook is executed.

Signed-off-by: Simon Ruderich <simon@ruderich.org>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -22,8 +22,10 @@ changed via the `core.hooksPath` configuration variable (see
 linkgit:git-config[1]).
 
 Before Git invokes a hook, it changes its working directory to either
-the root of the working tree in a non-bare repository, or to the
-$GIT_DIR in a bare repository.
+$GIT_DIR in a bare repository or the root of the working tree in a non-bare
+repository. An exception are hooks triggered during a push ('pre-receive',
+'update', 'post-receive', 'post-update', 'push-to-checkout') which are always
+executed in $GIT_DIR.
 
 Hooks can get their arguments via the environment, command-line
 arguments, and stdin. See the documentation for each hook below for