More cleanup and docs

[edsko]

We now have 100% Haddock coverage in grpc-spec:

Haddock coverage:
 100% ( 16 / 16) in 'Network.GRPC.Spec.Util.HKD'
 100% ( 10 / 10) in 'Network.GRPC.Spec.Util.Parser'
 100% (161 /161) in 'Network.GRPC.Spec'
 100% ( 50 / 50) in 'Network.GRPC.Spec.Serialization'

There are some link destinations not found, but they are either not exported for a reason (e.g. validDecoration), or they are generic representations, things Rep_RequestHeaders_. The latter is somewhat annoying in the haddock output as there are a lot of them, but not sure how to fix that.