You can think of await as “pausing” the async method until that operation is complete. As a special case, if the operation is already completed (or is extremely fast), then the await will not “pause” the method; it will continue executing immediately.
So in this case (assuming that WriteStartDocumentAsync is not already completed), await will pause the method and return an uncompleted task to the caller. Note that the Task returned by an async method represents that method; when the method completes, then that Task is completed.
Eventually, WriteStartDocumentAsync will complete, and that will schedule the rest of the async method to continue running. In this case, it’ll execute the next part of the method until the next await, when it gets paused again, etc. Eventually, the async method will complete, which will complete the Task that was returned to represent that method.
For more information, I have an async/await intro on my blog.