Ejection documentation should call out ephemeral differences

This ticket was raised after triage of MB-59842. That ticket was raised because it was observed that the bucket memory fell below the low-water mark for an ephemeral, auto-delete bucket.

The behaviour is correct, the issue is that expecting/using the low-water mark as the "target" for ejection is incorrect. An ephemeral bucket computes a "pageable" low-water mark as the target for ejection. The pageable low-water mark is the active share of the usual low-water mark. E.g. if a bucket:node has a 50/50 active/replica ratio, then pageable low-water mark is 50% of the low-water mark.

This ticket mainly impacts the following section:

https://docs.couchbase.com/server/current/learn/buckets-memory-and-storage/memory.html#ejection

But i've not looked for other places we may wish to improve.