Finish documentation re. v3 update

I need to re-scope this issue when we have succeeded in removing CustomElementsConfigGenerator, i.e. when 📌 Stop auto-generated entity-ce-display config Active is done.

By then, I will likely already have re-assembled/voiced an opinion about BC / update path, so I'll be better able to see if something is still missing.

so I guess we can simply drop all the CE-auto-generation now + have displayCE config be auto-generated and used on the fly by default. as BC, we could add a new flag in the displayCE config "forceAutoProcessing" which makes use of automatic processing for the whole entity, instead of applying the per-component configuration. Then everyone who wants BC can set this flag. We could even decide to expose it as some sort of setting in the UI later.

it seems that a conclusion we came already on a year ago: there is a pre-existing ticket for this: #3323558: Make configurable Auto option build Custom element same as existing processors do →

The changes that are by now already made re. the now-obsoleted #3323558: Make configurable Auto option build Custom element same as existing processors do → , make things more straightforward.

Retitling this ticket to be only about documenting the 'update path'. (Or rather: what exactly to do when you want the old v2 behavior back.)

Keeping this open until the checkbox for "force auto processing" is implemented in the UI.