Add cautions about incompatibility of backups across diferent platforms.

[Anderhar]

This week I reset the Wire app on my Android, expecting to restore the conversations from my iPad afterwards. Wish I had known in advance that it is just impossible...

And it's worth mentioning that even when trying to run a restoration from an incompatible file, the app did not explain what exactly went wrong, so I made multiple attempts until finally decided to check the documentation, where this article is also a bit difficult to find.

I think this is an important enough limitation to forewarn about with some insistence.