[Form] Compound option explained

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

@@ -3,6 +3,17 @@ 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).
+The compound option determines whether a form can have children,
+in other words is a parent form for other forms.
+
+In terms of data a compound form usually represents an object or an array.
+In terms of view a compound form might be a simple ``<div>`` container or
+a ``<form>`` element (the whole form).
+
+A non compound form usually represents an object property or an array value
+and is rendered with actual form elements: ``<input>`` (``TextType``, ``HiddenType``),
+``<textarea>`` (``TextareType``) and ``<select>`` (``ChoiceType``).
+
+A interesting case is the ``ChoiceType``. With ``expanded=false`` it is rendered as
+a ``<select>`` form element and thus it is not a compound form. With ``expanded=true``
+the ``ChoiceType`` is rendered as a set of radios or checkboxes and becomes a compound form.