types.ConsensusParams: document the maximum hard limit on BlockSizeParams.MaxBytes 

I was just reading through 
https://github.com/tendermint/tendermint/blob/d4634dc6832a7168c2536e7c71bfba4a8ca857be/types/params.go#L82

in order to write tests and noticed that the maximum hard limit is an unexposed constant that doesn't seem to be documented anywhere. Perhaps we should document this hardlimit.