PoS spec fixes

[brentstone]

Upgrading the cubic slashing description in the Namada specs. Cleaning some other lazy text fragments in other parts of the PoS specs.

TODO:

• may need to think more precisely about how we handle a validator who commits two infraction within an epoch of each other, as the sum total voting power fraction could exceed 1/3 if the validator voting power is large enough.
• #815