Clarify tombstone purge interval when shared bucket access is false

Hi,

I found unclear the purge interval for Mobile tombstones when enable_shared_bucket_access: false (because I ignored they were retained indefinitely by default).

As proposed by Adam Fraser, is it possible to change the following sentence :

Tombstones can be automatically purged from the bucket by setting a server expiry on tombstone documents.

by :

Tombstones are not automatically purged from the bucket.  Tombstones can be purged by setting a server expiry on tombstone documents.