firejail.config: add warning about allow-tray

According to netblue30#4053, `dbus-user.talk org.kde.StatusNotifierWatcher` is
unsafe and allows escaping the sandbox, but it is required by multiple
programs for tray functionality.  Users may not be aware of this (for
example, see netblue30#4508), so add a warning about it.

Note: allow-tray was added on commit c86cae2 ("Add new condition
ALLOW_TRAY", 2021-09-04) / PR netblue30#4510.

etc/firejail.config

@@ -2,7 +2,8 @@
 # keyword-argument pairs, one per line. Most features are enabled by default.
 # Use 'yes' or 'no' as configuration values.
 
-# Allow programs to display a tray icon
+# Allow programs to display a tray icon (WARNING: allows escaping the sandbox;
+# see https://github.com/netblue30/firejail/discussions/4053)
 # allow-tray no
 
 # Enable AppArmor functionality, default enabled.