minor #8339 [DI] fix flash message generation (xabbuh, javiereguiluz)

This PR was merged into the 3.3 branch.

Discussion
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[DI] fix flash message generation

see #8224 (comment)

Commits
-------

cba0478 Refactored the code
cdcb5c8 fix flash message generation

Author: javiereguiluz

service_container.rst

@@ -370,9 +370,8 @@ made. To do that, you create a new class::
                 ->addPart(
                     'Someone just updated the site. We told them: '.$happyMessage
                 );
-            $this->mailer->send($message);
-
-            return $happyMessage;
+            
+            return $this->mailer->send($message) > 0;
         }
     }
 
@@ -386,8 +385,10 @@ you can use the service immediately::
     {
         // ...
 
-        $message = $siteUpdateManager->notifyOfSiteUpdate();
-        $this->addFlash('success', $message);
+        if ($siteUpdateManager->notifyOfSiteUpdate()) {
+            $this->addFlash('success', 'Notification mail was sent successfully.');
+        }
+        
         // ...
     }