Clarify user prompt handling for switch to parent frame when already top-level browsing context

[whimboo]

With my recent changes on #1556 we do not run a check for user prompts when the current browsing context is already the top-level browsing context.

The question is if this is fine given that the command would be a no-op, or if we should absolutely check for an open user prompt.

@AutomatedTester and @jgraham your feedback is welcome. I'm happy to submit an update and appropriate wdspec tests.