feature #15063 Update docs relating to translation extraction (natewiebe13)

This PR was squashed before being merged into the 5.3-dev branch.

Discussion
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Update docs relating to translation extraction

Update translation extraction docs based on changes from symfony/symfony#40229

Fixes #15061

Commits
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77089fa Update docs relating to translation extraction

Author: wouterj

translation.rst

@@ -292,6 +292,8 @@ To manage these situations, Symfony follows the `ICU MessageFormat`_ syntax by
 using PHP's :phpclass:`MessageFormatter` class. Read more about this in
 :doc:`/translation/message_format`.
 
+.. _translatable-objects:
+
 Translatable Objects
 --------------------
 
@@ -462,6 +464,9 @@ The ``translation:update`` command looks for missing translations in:
   :ref:`twig.paths <config-twig-paths>` config options);
 * Any PHP file/class that injects or :doc:`autowires </service_container/autowiring>`
   the ``translator`` service and makes calls to the ``trans()`` method.
+* Any PHP file/class stored in the ``src/`` directory that creates
+  :ref:`translatable-objects` using the constructor or the ``t()`` method or calls
+  the ``trans()`` method.
 
 .. _translation-resource-locations:
 

translation/debug.rst

@@ -19,9 +19,10 @@ command helps you to find these missing or unused translation messages templates
 
 .. caution::
 
-    The extractors can't find messages translated outside templates, like form
-    labels or controllers. Dynamic translations using variables or expressions
-    in templates are not detected either:
+    The extractors can't find messages translated outside templates (like form
+    labels or controllers) unless using :ref:`translatable-objects` or calling
+    the ``trans()`` method on a translator. Dynamic translations using variables
+    or expressions in templates are not detected either:
 
     .. code-block:: twig