docs(router): Clarify how to clear secondary routes - #40272

The example for clearing secondary outlets currently only works because
the named outlet appears at the root of the application's route config.
If developers follow this example with an outlet that is not at the
root level, they will not be able to close the outlet.
This commit updates the example to provide a more robust way
of clearing the outlet from the activated outlet component
as well as providing a warning about how the commands are applied.

Lastly, there is a small bit of guidance provided for developers who
might want to explore the ability to close an outlet from any location
in the app, without needing to be aware of the activated route.
An example of this can be found here:
https://stackblitz.com/edit/close-outlet-from-anywhere

resolves #13523

Question to reviewers:
I wonder if I should omit the last bit about closing the outlet using navigateByUrl because it goes beyond the common use-cases of just closing the outlet from either the parent that opened it or the activated child itself.
It's quite an advanced topic and complicated to explain but may still be something that devs want to do.