Update MongoDB Connector documentation to reflect the change to Mongo Change Streams

The documentation on the MongoDB connector was not fully updated to reflect the change in Debezium 1.8 from oplog to change streams.

In particular, this section which explains how the connector actually works is really lacking in v1.8.

This is important because the change makes me question how Debezium can ensure that each oplog row becomes an event: in 1.7, the position in the oplog was saved to track this. In 1.8, we are presumably relying on Mongo to take care of this - but I want to know for sure.