minor #20360 [Translation] document the translation:extract command's…

… --prefix option (xabbuh)

This PR was merged into the 5.4 branch.

Discussion
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[Translation] document the translation:extract command's --prefix option

backport from #20352

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942396b document the translation:extract command's --prefix option

translation.rst

@@ -460,6 +460,15 @@ The ``translation:extract`` command looks for missing translations in:
     Support for extracting Translatable objects has been introduced in
     Symfony 5.3.
 
+By default, when the ``translation:extract`` command creates new entries in the
+translation file, it uses the same content as both the source and the pending
+translation. The only difference is that the pending translation is prefixed by
+``__``. You can customize this prefix using the ``--prefix`` option:
+
+.. code-block:: terminal
+
+    $ php bin/console translation:extract --force --prefix="NEW_" fr
+
 .. _translation-resource-locations:
 
 Translation Resource/File Names and Locations