[DOCS] Document setting eligibility for `elasticsearch-keystore add` and `elasticsearch-keystore add-file`

As of this writing, our [secure settings](https://www.elastic.co/guide/en/elasticsearch/reference/current/secure-settings.html) docs do not indicate that `elasticsearch-keystore add` and `elasticsearch-keystore add-file` should only be used for specific settings.

As a result, some users may assume that keystore can be used for any setting wished.