minor #9254 [Form] Event form modification improvements (julienfalque)

This PR was merged into the 2.7 branch.

Discussion
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[Form] Event form modification improvements

* `FormEvents::SUBMIT` *does* allow to modify the form (tested on Symfony 3.3 but looking at the code, this seems to be true for 2.7 as well);
* `FormEvents::POST_SUBMIT` allows to modify the parent form, this was not explicit in the documentation so I thought it would be a nice addition.

Commits
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d40285b Event form modifications improvements

Author: javiereguiluz

form/dynamic_form_modification.rst

@@ -591,6 +591,11 @@ callbacks only because in two different scenarios, the data that you can use is
 available in different events. Other than that, the listeners always perform
 exactly the same things on a given form.
 
+.. tip::
+
+    The ``FormEvents::POST_SUBMIT`` event does not allow to modify the form
+    the listener is bound to, but it allows to modify its parent.
+
 One piece that is still missing is the client-side updating of your form after
 the sport is selected. This should be handled by making an AJAX call back to
 your application. Assume that you have a sport meetup creation controller::

form/events.rst

@@ -219,7 +219,8 @@ View data        Normalized data transformed using a view transformer
 
 .. caution::
 
-    At this point, you cannot add or remove fields to the form.
+    At this point, you cannot add or remove fields to the current form and its
+    children.
 
 .. sidebar:: ``FormEvents::POST_SUBMIT`` in the Form component