Document ViewsConfigUpdater

@dww mentioned on slack one area for possible improvement:

Yeah, it could potentially be refactored to not make it so hard to use. Eg this badness of the class trying to save changes that then trigger views_view_presave() instantiating the class again to redo the work, etc.

I think it may be out of scope for this issue, but one of the issues is also that views config updates often require fixture updates which is a whole layer of complexity since they are zipped.

Here is how ECA approaches a similar issue: https://git.drupalcode.org/project/eca/-/merge_requests/423/diffs#031db4...

Additional issues adding this now:
✨ Allow a custom HTML element to be selected for a grouping field Needs work
✨ Add support for tables with two headers in views table display Needs work

My comment is linked in the summary, but copying here for visibility:

However, I had to completely rewrite ViewsConfigUpdater::processEntityArgumentUpdate() to operate on entire View entities, instead of using all the per-handler plumbing we've got. To my great dismay, if you copy the existing patterns, ViewsConfigUpdater process functions make a bunch of changes to handler config, then try to save the view, but that instantiates another ViewsConfigUpdater object (this time, with deprecations enabled) to do all the work again. 🤯 So apparently, The Right Way™️ to update a view with all this plumbing is that your process function has to directly update the $view (which isn't even normally passed in to your process function if you think you're just dealing with handlers), not just fix the $handler. 🤦‍♂️

It's probably out of scope to actually fix that as part of an issue with the title "Document ...", but maybe we want to expand the scope here?

I'm ok with slightly expanding scope or making this more of a META.

Honestly, especially now that we are starting to validate config and deprecate it a more generic config updater may be necessary.

@jurgenhaas wrote an updater too to help with ECA token changes. 📌 Provide post update helper function to rename tokens Fixed

Having some generic update notification / modification almost like a hook would be helpful for core and for contrib to keep configuration up to date.