Add event handling and control clipping documentation (#13325)
Summary of the issue:
As a developer, it would be helpful to have a summary of wxWidgets behaviour that is known to cause bugs in NVDA.
Some wxWidgets behaviour is complicated, unexpected, or has changed over time, leading to bugs in NVDA.
Description of how this pull request fixes the issue:
This PR adds documentation for fixing missing/clipping controls, as well as handling events.
