document app.Storage() from `storage` API docs

[dolanor]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I had no idea how to get a correct path for an app in Android (which must have been in the app directory) to be able to create/read files.

@andydotxyz told me to look at fyne.App.Storage()

Is it possible to construct a solution with the existing API?

No response

Describe the solution you'd like to see.

It would be good to know from the storage package documentation that platform specific helpers exist in the fyne package.

[andydotxyz]

Sorry, just looking to clarify - which platform specific helpers were missed in the docs?
The main page has a pretty extensive coverage of the APIs https://docs.fyne.io/api/v2/storage/package/ - but I'd be happy to add or make them more visible when I understand where it fell short.