ICS28: Reflect that downtime slashing is not allowed after doublesign slashing in spec

[danwt]

See mirror issue

• https://github.com/cosmos/interchain-security/issues/220

Problem: the behavior allowed by the spec is different from the impl although we established that the impl has the correct design, so we should update the spec.

Note the spec allows more slashing to occur than the impl.

[manofthemooon]

The problem stems from a discrepancy between the behavior specified in the documentation and the actual implementation. While the implementation aligns with the intended design, the specification permits more slashing to occur than what is implemented.

[danwt]

Thanks that's fine