minor #7767 Added caution block under delete_empty to warn the developer when he try to activate delete_empty for collections of compound forms (murilolobato, javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Added caution block under delete_empty to warn the developer when he try to activate delete_empty for collections of compound forms

Commits
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ce30e47 Applied reviewer suggestion
52c0a94 Minor fix
de25ede Minor reword
ca04555 Improved format of message and added missing information.
b9e375c Added caution block under delete_empty to warn the developer when he try to activate delete_empty for collections of compound forms.

Author: xabbuh

reference/forms/types/collection.rst

@@ -273,6 +273,15 @@ form you have to set this option to true. However, existing collection entries
 will only be deleted if you have the allow_delete_ option enabled. Otherwise
 the empty values will be kept.
 
+.. caution::
+
+    The ``delete_empty`` option only removes items when the normalized value is
+    ``null``. If the nested `type`_ is a compound form type, you must either set
+    the ``required`` option to ``false`` or set the ``empty_data`` option to 
+    ``null``. Both of these options can be set inside `options`_. See
+    :ref:`form's empty_data option <reference-form-option-empty-data>` to learn
+    why this is necessary.
+
 options
 ~~~~~~~
 

reference/forms/types/form.rst

@@ -80,6 +80,8 @@ option on the form.
 
 .. include:: /reference/forms/types/options/data_class.rst.inc
 
+.. _reference-form-option-empty-data:
+
 .. include:: /reference/forms/types/options/empty_data.rst.inc
     :end-before: DEFAULT_PLACEHOLDER