This Cursor Snapshot section on read isolation refers to behavior that only existed in MMAP.
This paragraph should be deleted since it is no longer applicable:
If your collection has a field or fields that are never modified, you can use a unique index on this field or these fields so that the query will return each document no more than once. Query with hint() to explicitly force the query to use that index.
I think we should replace it with this, instead:
Queries that use unique indexes can, in some cases, return duplicate values. If a cursor using a unique index interleaves with a delete and insert of documents sharing the same unique value, the cursor may return the same unique value twice, but from different documents.