Explained a more simple method of dynamic form handling available since
https://github.com/symfony/symfony/pull/8827
Merge branch 'dynamic_forms_after_PR8827' of github.com:Burgov/symfony-docs into Burgov-dynamic_forms_after_PR8827

Conflicts:
	cookbook/form/dynamic_form_modification.rst
[#2927] Tweaks to form events changes
Merge remote-tracking branch 'origin/2.2' into 2.2