Don't implement interfaces extended by another that is also implemented

PHP<7.4 does not allow this.
Instead, for clarity,
add a DocBlock comment stating which additional interfaces should be
implemented that are not explicitly listed in the `implements` section.

When our minimum PHP version becomes 7.4 or above, we can revisit this.

Author: JakeQZ

src/CSSList/CSSList.php

@@ -30,8 +30,11 @@
  * `RuleSet`s as well as other `CSSList` objects.
  *
  * It can also contain `Import` and `Charset` objects stemming from at-rules.
+ *
+ * Note that `CSSListItem` extends both `Commentable` and `Renderable`,
+ * so those interfaces must also be implemented by concrete subclasses.
  */
-abstract class CSSList implements Commentable, CSSListItem, Renderable
+abstract class CSSList implements CSSListItem
 {
     /**
      * @var list<Comment>

src/Property/AtRule.php

@@ -8,7 +8,11 @@
 use Sabberworm\CSS\CSSList\CSSListItem;
 use Sabberworm\CSS\Renderable;
 
-interface AtRule extends Commentable, CSSListItem, Renderable
+/**
+ * Note that `CSSListItem` extends both `Commentable` and `Renderable`,
+ * so concrete classes implementing this interface must also implement those.
+ */
+interface AtRule extends CSSListItem
 {
     /**
      * Since there are more set rules than block rules,

src/RuleSet/RuleSet.php

@@ -22,8 +22,11 @@
  *
  * If you want to manipulate a `RuleSet`, use the methods `addRule(Rule $rule)`, `getRules()` and `removeRule($rule)`
  * (which accepts either a `Rule` or a rule name; optionally suffixed by a dash to remove all related rules).
+ *
+ * Note that `CSSListItem` extends both `Commentable` and `Renderable`,
+ * so those interfaces must also be implemented by concrete subclasses.
  */
-abstract class RuleSet implements Commentable, CSSListItem, Renderable
+abstract class RuleSet implements CSSListItem
 {
     /**
      * the rules in this rule set, using the property name as the key,