Document what "storage compatibility" means for `set_code_hash`

[cmichi]

Follow-up to https://substrate.stackexchange.com/questions/2352/old-and-new-storage-compatibility-inkenvset-code-hash.

We should document what "storage compatibility" actually means. As I wrote in the answer for the StackExchange question it's the same requirements as in the OpenZeppelin docs.

This explanation of "storage compatibility" should also be linked on https://github.com/paritytech/ink/tree/master/examples/upgradeable-contracts.