submitConfigurationForm not getting called

Problem/Motivation

I created a style option plugin, and was about to change the structure to simplify when saving it.

I overrode submitConfigurationForm, but it was never called after clearing caches.

I modified the code for the background image plugin, and it is also not getting called.

Steps to reproduce

* Create a style options yml file and assign it to a layout or paragraph.
* Add code on submitConfigurationForm to kill or log a message
* Edit or creade a node, add the image, verify that it doesn't get called.

Proposed resolution

* Document what is the right way to implement a submit handler.