improve docs app-install-banners/native

[trollepierre]

Page Affected: https://developers.google.com/web/fundamentals/app-install-banners/native,

What needs to be done? The doc is not clear, not working, with a lot of misconfusion between PWA and native apps

https://stackoverflow.com/questions/55838392/how-exactly-are-we-supposed-to-implement-the-native-app-install-prompt-for-chrom

Please provide description of what's needs to be done here. Please be sure to include the link to the original page and any supporting links/data.

I guess you should read the stackoverflow page that helps this page