minor #17092 [Form] Better explaining what getParent actually does (ThomasLandauer)

This PR was merged into the 5.4 branch.

Discussion
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[Form] Better explaining what `getParent` actually does

Main reason for changing this is to stop presenting `getParent()` of being the "default" (or normal) way of doing it.

* I removed the "PHP extension" note-box, cause the (new) text for `getParent()` makes it clear what this does.
* I moved `finishView()` one paragraph up (didn't change anything)
* Moved all `getParent`-related stuff into its paragraph, and moved it to the top (since it's the most important question in the beginning)

Commits
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1c9b524 Better explaining what `getParent` actually does

Author: javiereguiluz

form/create_custom_field_type.rst

@@ -55,19 +55,11 @@ By convention they are stored in the ``src/Form/Type/`` directory::
         }
     }
 
-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.
+``getParent()`` tells Symfony to take ``ChoiceType`` as a starting point,
+then ``configureOptions()`` overrides some of its options. (All methods of the
+``FormTypeInterface`` are explained in detail later in this article.)
 The resulting form type is a choice field with predefined choices.
 
-.. note::
-
-    The PHP class extension mechanism and the Symfony form field extension
-    mechanism are not the same. The parent type returned in ``getParent()`` is
-    what Symfony uses to build and manage the field type. Making the PHP class
-    extend from ``AbstractType`` is only a convenient way of implementing the
-    required ``FormTypeInterface``.
-
 Now you can add this form type when :doc:`creating Symfony forms </forms>`::
 
     // src/Form/Type/OrderType.php
@@ -123,44 +115,34 @@ convenient to extend instead from :class:`Symfony\\Component\\Form\\AbstractType
         // ...
     }
 
-When a form type doesn't extend from another specific type, there's no need to
-implement the ``getParent()`` method (Symfony will make the type extend from the
-generic :class:`Symfony\\Component\\Form\\Extension\\Core\\Type\\FormType`,
-which is the parent of all the other types).
-
 These are the most important methods that a form type class can define:
 
 .. _form-type-methods-explanation:
 
+``getParent()``
+    When returning a (fully-qualified) class name here, Symfony will call each
+    method of that type (i.e. ``buildForm()``, ``buildView()``, etc.) and all its
+    type extensions, before calling the corresponding method of your custom type.
+    This is probably a good idea if you're just changing some details of an
+    existing type. To start from scratch, just omit ``getParent()``.
+
 ``buildForm()``
     It adds and configures other types into this type. It's the same method used
     when :ref:`creating Symfony form classes <creating-forms-in-classes>`.
 
 ``buildView()``
     It sets any extra variables you'll need when rendering the field in a template.
 
-``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. 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 ``buildView()`` instead.
 
-``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.
+``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. Options are inherited from parent types and parent type
+    extensions, but you can create any custom option you need.
 
 Defining the Form Type
 ~~~~~~~~~~~~~~~~~~~~~~