minor #14588 [Form] Added explicit getParent() call in types inheritance mechanism (HeahDude, wouterj)

This PR was merged into the 4.4 branch.

Discussion
----------

[Form] Added explicit `getParent()` call in types inheritance mechanism

Continues #13488

Most of the original PR was already fixed in a slight rewrite of this article, but I think adding `getParent()` docs is still important.

Commits
-------

97c94d7 Updated getParent() description with the why/when (thanks javier!)
68813a0 [Form] Added explicit `getParent()` call in types inheritance mechanism

Author: javiereguiluz

form/create_custom_field_type.rst

@@ -55,13 +55,10 @@ By convention they are stored in the ``src/Form/Type/`` directory::
         }
     }
 
-The ``configureOptions()`` method, which is explained later in this article,
-defines the options that can be configured for the form type and sets the
-default value of those options.
-
-The ``getParent()`` method defines which is the form type used as the base of
-this type. In this case, the type extends from ``ChoiceType`` to reuse all of
-the logic and rendering of that field type.
+The methods of the ``FormTypeInterface`` are explained in detail later in
+this article. Here, ``getParent()`` method defines the base type
+(``ChoiceType``) and ``configureOptions()`` overrides some of its options.
+The resulting form type is a choice field with predefined choices.
 
 .. note::
 
@@ -144,13 +141,27 @@ These are the most important methods that a form type class can define:
 
 ``configureOptions()``
     It defines the options configurable when using the form type, which are also
-    the options that can be used in ``buildForm()`` and ``buildView()`` methods.
+    the options that can be used in ``buildForm()`` and ``buildView()``
+    methods. Options are inherited from parent types and parent type
+    extensions, but you can create any custom option you need.
 
 ``finishView()``
     When creating a form type that consists of many fields, this method allows
     to modify the "view" of any of those fields. For any other use case, it's
     recommended to use instead the ``buildView()`` method.
 
+``getParent()``
+    If your custom type is based on another type (i.e. they share some
+    functionality) add this method to return the fully-qualified class name
+    of that original type. Do not use PHP inheritance for this.
+    Symfony will call all the form type methods (``buildForm()``,
+    ``buildView()``, etc.) of the parent type and it will call all its type
+    extensions before calling the ones defined in your custom type.
+
+    By default, the ``AbstractType`` class returns the generic
+    :class:`Symfony\\Component\\Form\\Extension\\Core\\Type\\FormType`
+    type, which is the root parent for all form types in the Form component.
+
 Defining the Form Type
 ~~~~~~~~~~~~~~~~~~~~~~