minor symfony#12046 [Form] Mention that by_reference only works in English (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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[Form] Mention that by_reference only works in English

Fixes symfony#6681.

Commits
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f81f76c [Form] Mention that by_reference only works in English

Author: OskarStark

form/form_collections.rst

@@ -445,6 +445,12 @@ you will learn about next!).
     otherwise the form will still use ``setTag()`` even if ``by_reference`` is ``false``.
     You'll learn more about the ``removeTag()`` method later in this article.
 
+.. caution::
+
+    Symfony can only make the plural-to-singular conversion (e.g. from the
+    ``tags`` property to the ``addTag()`` method) for English words. Code
+    written in any other language won't work as expected.
+
 .. sidebar:: Doctrine: Cascading Relations and saving the "Inverse" side
 
     To save the new tags with Doctrine, you need to consider a couple more