Documentation: Limitation on the number of in-flight `VideoFrame`s stalling capture.

[OutOfTheVoid]

When a stream is created, it is configured with a certain "buffer count", which is the maximum number of capture frames that can remain in-flight at a time. When this limit is reached, a capture stream will stall until a buffer becomes available. In CrabGrab, that means drop()ing a VideoFrame

We need to better document this behavior. It would be good to have a FAQ or Useful Info section in the crate root documentation that goes over this and other screen capture quirks.