incorrect docs for EntityContentBase 'translations' configuration option

Input from @mikelutz on Slack:

> On the technical side, it does 2 things. It adds the langcode as a destination id, and on the import side makes sure that you are creating or updating a translation of an existing entity, instead of trying to create a new entity or attempting to change the base language of an existing entity.

> It’s real purpose is to add the langcode as an ID and to update a translation instead of trying to update the base entities langcode. As such, with the old style migrations, you would migrate the base translations first, without setting translations to true, and then migrate the translations in a separate migration that looks up the node ids of the original translations and wants to update/add a translation of that, so we would set translations:true for that one.

> It is possible to migrate all base languages and translations in one shot with a source that has the right configuration using one migration with translations:true, but core didn’t do it that way.

Some of the above needs to be picked out and used for fixing the docs.

Core maintainers: please add credit for @mikelutz.

smustgrave → credited mikelutz → .

Re-ran test failures and as expected random.

Reading the change and to me in reads just fine. Believe should be good.

I don't think this should remove the point that 'translations' "indicates if the entity is translatable".

@quietone, I'll make the necessary changes suggested in the comments.

@quietone
I made the suggested changes in the documentation
Please take a look

I also rebased the fork to the latest changes
Please take a look

Rebased the fork to the latest changes

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