Merge pull request #773 from tendermint/docs-staging

docs improvements

types/block.go

@@ -169,7 +169,7 @@ type Header struct {
 }
 
 // Hash returns the hash of the header.
-// NOTE: hash is nil if required fields are missing.
+// Returns nil if ValidatorHash is missing.
 func (h *Header) Hash() data.Bytes {
 	if len(h.ValidatorsHash) == 0 {
 		return nil