Differences with show() between backends

I was just testing out the examples from the docs and figured I would put some quick findings here while running on a Mac.

* Documentation: Change `subplot_module` to `subplot_mosaic` in a couple of places in the code samples
* `qt5agg`: mg.show() does not exit after closing both figures, the event loop keeps running
* `macosx`: mg.show() closing one figure closes all of the figures, as if the closeWindow gets sent to every figure, not just the one clicked.
* `tkagg`: In the second `fr.show()` call in the examples I get this error: `_tkinter.TclError: can't invoke "wm" command: application has been destroyed` (macosx seems to get through this and the qt5agg I don't think ever even gets here due to the event loop not quiting after the first show() as mentioned above)

Start by reproducing the documented examples on macOS with the qt5agg, macosx, and tkagg backends, focusing on each reported show() and figure-closing behavior. Review the examples containing subplot_module and the backend-specific show/close entry points mentioned in the report. Done means the samples use subplot_mosaic and each backend handles the reported event-loop, window-closing, and second-show cases without the stated errors.

Written by the indexing model from the issue text.