Document that the same password shouldn't be reused to protect multiple keys

[defuse]

We need to document that the user must never protect two keys with the same password if they need integrity, since the ciphertexts can just be swapped.

[defuse]

If you wanted integrity, and wanted to use the same password, then you'd have to "name" the keys. The name of a key would be some sort of cryptographic domain separation parameter.

[defuse]

Done. in #226.