githooks.txt: amend dangerous advice about 'update' hook ACL

Any ACL you implement via an 'update' hook isn't actual access control
if the user has login access to the machine running git, because they
can trivially just build their own version of Git which doesn't run the
hook.

Change the documentation to take this dangerous edge case into account,
and remove the mention of the advice originating on the mailing list,
the users reading this don't care where the idea came up.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -274,9 +274,11 @@ does not know the entire set of branches, so it would end up
 firing one e-mail per ref when used naively, though.  The
 <<post-receive,'post-receive'>> hook is more suited to that.
 
-Another use suggested on the mailing list is to use this hook to
-implement access control which is finer grained than the one
-based on filesystem group.
+In an environment that restricts the users' access only to git
+commands over the wire, this hook can be used to implement access
+control without relying on filesystem ownership and group
+membership. See linkgit:git-shell[1] for how you might use the login
+shell to restrict the user's access to only git commands.
 
 Both standard output and standard error output are forwarded to
 'git send-pack' on the other end, so you can simply `echo` messages