minor #6170 change translation getMessages() to getCatalogue() (snoek09)

This PR was squashed before being merged into the 2.8 branch (closes #6170).

Discussion
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change translation getMessages() to getCatalogue()

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.8
| Fixed tickets | #6137

I'm assuming here that the structure of the ``$messages`` variable didn't change.

Commits
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06ca405 change translation getMessages() to getCatalogue()

components/translation/usage.rst

@@ -376,7 +376,11 @@ In case you want to use the same translation catalogue outside your application
 (e.g. use translation on the client side), it's possible to fetch raw translation
 messages. Just specify the required locale::
 
-    $messages = $translator->getMessages('fr_FR');
+    $catalogue = $translator->getCatalogue('fr_FR');
+    $messages = $catalogue->all();
+    while ($catalogue = $catalogue->getFallbackCatalogue()) {
+        $messages = array_replace_recursive($catalogue->all(), $messages);
+    }
 
 The ``$messages`` variable will have the following structure::