Merge branch '5.2' into 5.3

* 5.2:
  [5.2] Add missing `@return` annotations
  [4.4] Add missing `@return` annotations
  Handle lock with long key
  #42227 update phpdoc to recommend createIndex over ensureIndex

Author: nicolas-grekas

Transport/Smtp/SmtpTransport.php

@@ -340,6 +340,9 @@ private function checkRestartThreshold(): void
         $this->restartCounter = 0;
     }
 
+    /**
+     * @return array
+     */
     public function __sleep()
     {
         throw new \BadMethodCallException('Cannot serialize '.__CLASS__);