add details of consensus fault slashing

[hunjixin]

No description provided.

[hunjixin]

filecoin-project/venus#4586
filecoin-project/venus#4588

after the last consensus fault, we summarized the specific consensus fault slashing.
and have a suggestion, check whether the miner of the block and the message receiver address are the same after step 5. when verifying the block signature, used the miner address in the blocks. this might be clearer.

[hunjixin]

@jennijuju It may be better to put this impl in the spec-actors
.