State RFC: clarify difficulty
The difficulty validation RFC will introduce a definition of
per-block difficulty. Make it clear that the state RFC
definition is cumulative difficulty.
@@ -39,9 +39,10 @@ state service.
* **chain state**: The state of the ledger after application of a particular
sequence of blocks (state transitions).
-* **difficulty**: The cumulative proof-of-work from genesis to the chain tip.
+* **cumulative difficulty**: The cumulative proof-of-work from genesis to the
+ chain tip.
-* **best chain**: The chain with the greatest difficulty. This chain
+* **best chain**: The chain with the greatest cumulative difficulty. This chain
represents the consensus state of the Zcash network and transactions.
* **side chain**: A chain which is not contained in the best chain.
