update some comments

types/block.go

@@ -124,7 +124,7 @@ func (b *Block) MakePartSet(partSize int) *PartSet {
 }
 
 // HashesTo is a convenience function that checks if a block hashes to the given argument.
-// A nil block never hashes to anything, and nothing hashes to a nil hash.
+// Returns false if the block is nil or the hash is empty.
 func (b *Block) HashesTo(hash []byte) bool {
 	if len(hash) == 0 {
 		return false