SQL Server 2012 - Frequent SELECT and UPDATE on bit column. Index doesn't help
An index on the bit column isn't going to help at all because of selectivity. You should consider a filtered index:

CREATE INDEX unindexed ON dbo.Documents(documentId) WHERE IsIndexed = 0;


You should also include an ORDER BY documentId in your SELECT.

Though you should test this in a staging environment because it may help your SELECT but may offset that with a more expensive UPDATE.