Document boolean frame masks, including NumPy arrays, in the User Guide. #445

The User Guide describes frame selection with start, stop, and step, for example in the [Quick start guide] and [“Writing your own trajectory analysis”]. It does not explain how to select frames with boolean masks passed to run(frames=...), including NumPy boolean arrays. A short explanation of how these masks select frames would be helpful.

This is related to MDAnalysis/mdanalysis#5476, which fixes the handling of NumPy boolean frame masks.