Docs for SERVER-37786: Reject change streams which modify or project-out the resume token

Description

Description:

As of this commit, any $changeStream pipeline that either removes or modifies the _id field will fail with an exception. This is because the _id field of each $changeStream event contains the resume token for that event; if it is modified or removed, then it becomes impossible to resume the stream from that point.

Engineering Ticket Description:

For example, this pipeline:

db.x.watch([{$replaceRoot: {newRoot: "$fullDocument"}}])

Such a pipeline is incompatible with the drivers protocol, and will fail to resume in the face of a network outage. It is not intended to be supported, so we should ban it to provide a more predictable failure scenario. Now that we have completed SERVER-35904, it shouldn't be very difficult to verify the _id is preserved by the pipeline.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)