docs: update subscriptions-external.md

[Keropon]

Added some extra explanations when using VueFire alongside the Pinia store.

The old docs had an implementation that didn't work properly when loading everything on a new page/route, as the data didn't propagate to the ref(), This implementation should hopefully work.

[posva]

Thanks for the PR but the code should work the way it is written 🤔 Do you have a reproduction?

[Keropon]

With 'reproduction' do you mean a project in which the docs' code didn't work? Or one in which the code of the PR works? Let me know and I'll try to make a demo

[posva]

a boiled down project in which to show an existing broken behavior with the current docs