document the gotcha about changing the webhook target on irc notif

I got bit by this and tried to send to `?channel=#tpo-admin`, which
obviously fails because # is a special character. Easier to just skip
the pound sign altogether.

howto/gitlab.md

@@ -269,8 +269,10 @@ URL should be:
 ... and you should select the notifications you wish to see in
 `#tor-bots`. You can also enable notifications to other channels by
 adding more parameters to the URL, like (say)
-`?channel=%23tor-foo`. The parameters are documented the [KGB
-documentation](https://salsa.debian.org/kgb-team/kgb/-/wikis/usage).
+`?channel=tor-foo`. Important note: do not try to put the `#` in
+the channel name, or if you do, URL-encode it (e.g. like `%23tor-foo`),
+otherwise this will silently fail to change the target channel. Other
+parameters are documented the [KGB documentation](https://salsa.debian.org/kgb-team/kgb/-/wikis/usage).
 
 Note that GitLab admins might be able to configure [system-wide
 hooks](https://gitlab.torproject.org/help/system_hooks/system_hooks) in [the admin section](https://gitlab.torproject.org/admin/hooks), although it's not entirely clear