Metadata Purge Interval in Auto Compactions sections should refer to Epehemrals

In the following section Metadata Purge Interval it is discussed about Couchbase buckets only, and it's tombstone purge interval.
i think we should add a reference to how you can do it in Ephemeral buckets as well as a per bucket setting.

https://docs.couchbase.com/server/current/manage/manage-buckets/create-bucket.html#ephemeral-bucket-settings

Reporter: Roi Katz
E-mail: roi.katz@Couchbase.com