minor #22065 [DI] Fix PhpDumper generated doc block (nicolas-grekas)

This PR was merged into the 2.8 branch.

Discussion
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[DI] Fix PhpDumper generated doc block

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

58b3ee7 [DI] Fix PhpDumper generated doc block

src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php

@@ -628,7 +628,7 @@ EOF;
         }
 
         if ($definition->isAutowired()) {
-            $doc = <<<EOF
+            $doc .= <<<EOF
 
      *
      * This service is autowired.

src/Symfony/Component/DependencyInjection/Tests/Fixtures/php/services24.php

@@ -33,6 +33,9 @@ class ProjectServiceContainer extends Container
     /**
      * Gets the 'foo' service.
      *
+     * This service is shared.
+     * This method always returns the same instance of the service.
+     *
      * This service is autowired.
      *
      * @return \Foo A Foo instance