This came out of an investigation for SERVER-43889, where the reporter was misusing the ReplaceOneAsync method with retryable writes. In this case, they were trying to run multiple concurrent retryable writes on the same session before waiting for the previous write to complete.

The documentation doesn't mention the overloaded version of ReplaceOneAsync, which can take a session.

We should document the proper way to use this (and similar async methods?) with sessions since they can produce undesirable results otherwise.