[TASK] Add native return type for render* methods (#1166)

Part of #811

Author: oliverklee

CHANGELOG.md

@@ -34,7 +34,7 @@ Please also have a look at our
 - Use more native type declarations and strict mode
   (#641, #772, #774, #778, #804, #841, #873, #875, #891, #922, #923, #933, #958,
   #964, #967, #1000, #1044, #1134, #1136, #1137, #1139, #1140, #1141, #1145,
-  #1162, #1163)
+  #1162, #1163, #1166)
 - Add visibility to all class/interface constants (#469)
 
 ### Deprecated

src/CSSList/CSSList.php

@@ -408,10 +408,7 @@ public function __toString(): string
         return $this->render(new OutputFormat());
     }
 
-    /**
-     * @return string
-     */
-    protected function renderListContents(OutputFormat $outputFormat)
+    protected function renderListContents(OutputFormat $outputFormat): string
     {
         $result = '';
         $isFirst = true;

src/RuleSet/RuleSet.php

@@ -269,10 +269,7 @@ public function __toString(): string
         return $this->render(new OutputFormat());
     }
 
-    /**
-     * @return string
-     */
-    protected function renderRules(OutputFormat $outputFormat)
+    protected function renderRules(OutputFormat $outputFormat): string
     {
         $result = '';
         $isFirst = true;