minor #9283 [Form] Compound option explained (vudaltsov)

This PR was squashed before being merged into the 2.7 branch (closes #9283).

Discussion
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[Form] Compound option explained

Closes #5213

ping @HeahDude , @wouterj , @xabbuh

Commits
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977dfb9 [Form] Compound option explained

Author: javiereguiluz

reference/forms/types/options/compound.rst.inc

@@ -3,6 +3,22 @@ compound
 
 **type**: ``boolean`` **default**: ``true``
 
-This option specifies if a form is compound. This is independent of whether
-the form actually has children. A form can be compound but not have any
-children at all (e.g. an empty collection form).
+If ``true`` this option creates the form as "compound", meaning that it
+can contain children and be a parent of other forms.
+
+Most of the time you won't need to override this option.
+You might want to control for it when creating a custom form type
+with advanced rendering logic.
+
+In a view a compound form is rendered as a ``<div>`` container or
+a ``<form>`` element (the whole form is obviously a compound form).
+
+Non-compound forms are always leaves in a form tree, they cannot have children.
+
+A non-compound form is rendered as one of the html form elements: ``<input>``
+(``TextType``, ``FileType``, ``HiddenType``), ``<textarea>`` (``TextareaType``)
+or ``<select>`` (``ChoiceType``).
+
+An interesting case is the ``ChoiceType``. With ``expanded=false`` it is a non-compound form
+and is rendered as a ``<select>`` tag. With ``expanded=true`` the ``ChoiceType`` becomes a
+compound form and is rendered as a set of radios or checkboxes.