Fixed duplicate shares on vardiff difficulty update. Added more comments.

lib/pool.js

@@ -67,6 +67,9 @@ var pool = module.exports = function pool(options, authorizeFn){
             because of how likely it is for a miner to exhaust the nonce range
             before new work is sent.
 
+            Here is where stratum-python is bugged:
+            https://github.com/Crypto-Expert/stratum-mining/blob/master/mining/basic_share_limiter.py#L171-L178
+
             So lets only send a new difficulty, and the worker will use it when
             it receives a new job from when the block template updates.
             */