const-signal-or-slot: Improve readme

Clarify what's bad about const signals

docs/checks/README-const-signal-or-slot.md

@@ -3,7 +3,12 @@
 Warns when a signal or non-void slot is const.
 
 This aims to prevent unintentionally marking a getter as slot, or connecting to
-the wrong method. For signals, it's just pointless to mark them as const.
+the wrong method.
+
+For signals it's more of a minor issue. Prevents you from emitting signals from
+const methods, as these methods shouldn't change state, and a signal implies state
+was changed. Helps minimizing having global state (which is the only state you can
+change from a const method).
 
 Warns for the following cases: