Improve docs on Filesystem and Storage apis

Some improvements:

- Explain how Storage is handled in non web platforms. What does it use instead of localstorage
- Explain how Filesystem is handled in different platforms (I found out by trial that Filesystem on web it uses indexeddb)
- In the Filesystem docs it mentions cloud backed locations, but it doesnt mention how to use them. Is it one of the FilesystemDirectory directories?
- In the Filesystem docs, mention how to handle binary files. (this is working for me: https://github.com/ionic-team/capacitor/issues/31#issuecomment-492779894)

Thanks, and great work, Im really liking capacitor 🙂