Document restore feature states dependencies

[eedugon]

This is a side issue of #1391 (comment)

Feature states are introduced here: https://www.elastic.co/docs/deploy-manage/tools/snapshot-and-restore#feature-state

Restoring feature state is documented here: https://www.elastic.co/docs/deploy-manage/tools/snapshot-and-restore/restore-snapshot#restore-feature-state

We need to explain in the docs that some feature states must be restored in conjunction with others, for example "fleet" feature state, which depends on "security" and "kibana" feature states.

“Fleet” relies on the .fleet system indices, Elasticsearch security indices for API keys, and Kibana for saved objects. If these are three separate feature states they would all have to be restored for Fleet to have a consistent view of everything it needs.

For the dependencies we need to be a bit vague, providing the example of Fleet, because we don't know if there are other dependencies, so we cannot enumerate them.

I would also include the tip to obtain the list of features of a cluster using GET _features API call in the restore document. That's included in the feature states presentation, but in the restore document it would be welcome too.

cc: @shainaraskas & @stefnestor

[stefnestor]

Related example: "restoring the Kibana UI" might restore the kibana and security feature states to restore the API Keys of the (Security Detections) Kibana Rules.

So maybe on wording we decouple a 1to1 idea of feature to UI of overlapping name?