Document UITester.find_by_name and find_by_id do not take visibility into account

Spin off from this discussion issue: https://github.com/enthought/traitsui/issues/1541

Some developers reading `UITester.find_by_name("name")` may assume visibility to have been taken into account, while some may not. It would be useful to document:

- UI visibility is not taken into account by `UITester.find_by_name` and `UITester.find_by_id`. In other words, the UI element located might be hidden, but that would not prevent `find_by_name` and `find_by_id` from finding it.

It might also be useful to add:
- For testing UI visibility states, the returned value of `inspect(IsVisible())` can be asserted in tests.