[BUGFIX] Fix a type annotation in RuleSet (#1061)

This is the V8.x backport of #1051.

Author: oliverklee

src/RuleSet/RuleSet.php

@@ -23,7 +23,10 @@
 abstract class RuleSet implements Renderable, Commentable
 {
     /**
-     * @var array<string, Rule>
+     * the rules in this rule set, using the property name as the key,
+     * with potentially multiple rules per property name.
+     *
+     * @var array<string, array<int<0, max>, Rule>>
      */
     private $aRules;