Improve Documentation on Locale extensions

[Othko97]

Locale extension runtimes ony get partially installed if the base SDK is already installed, but the behaviour is not well documented and was certainly confusing to this new user. In particular flatpak-builder refuses to build with the partial install and I could find nothing in the documentation to explain or fix the issue.

[matthiasclasen]

In what way did flatpak-builder fail to build ?

[Othko97]

flatpak-builder kicked out to say that the Locale extension was only partially installed. This was confusing to me as I'd just installed it, I think it should be clearer in the docs that installing Locale extensions gives only a partial install and if you're looking to build locally with them you need to run flatpak update