minor #13725 Add phpdoc for SecurityFactoryInterface::getPosition (adrienbrault)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #13725).

Discussion
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Add phpdoc for SecurityFactoryInterface::getPosition

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

Commits
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bcb2e09 Add phpdoc for SecurityFactoryInterface::getPosition

src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/Factory/SecurityFactoryInterface.php

@@ -23,6 +23,12 @@ interface SecurityFactoryInterface
 {
     public function create(ContainerBuilder $container, $id, $config, $userProvider, $defaultEntryPoint);
 
+    /**
+     * Defines the position at which the provider is called.
+     * Possible values: pre_auth, form, http, and remember_me.
+     * 
+     * @return string
+     */
     public function getPosition();
 
     public function getKey();