Clarify CCoinsViewMemPool documentation.

Thanks to @sdaftuar for correcting my misunderstanding.

src/rpc/blockchain.cpp

@@ -991,7 +991,7 @@ UniValue gettxout(const JSONRPCRequest& request)
     if (fMempool) {
         LOCK(mempool.cs);
         CCoinsViewMemPool view(pcoinsTip, mempool);
-        if (!view.GetCoin(out, coin) || mempool.isSpent(out)) { // TODO: filtering spent coins should be done by the CCoinsViewMemPool
+        if (!view.GetCoin(out, coin) || mempool.isSpent(out)) {
             return NullUniValue;
         }
     } else {

src/txmempool.h

@@ -676,6 +676,13 @@ private:
 /** 
  * CCoinsView that brings transactions from a memorypool into view.
  * It does not check for spendings by memory pool transactions.
+ * Instead, it provides access to all Coins which are either unspent in the
+ * base CCoinsView, or are outputs from any mempool transaction!
+ * This allows transaction replacement to work as expected, as you want to
+ * have all inputs "available" to check signatures, and any cycles in the
+ * dependency graph are checked directly in AcceptToMemoryPool.
+ * It also allows you to sign a double-spend directly in signrawtransaction,
+ * as long as the conflicting transaction is not yet confirmed.
  */
 class CCoinsViewMemPool : public CCoinsViewBacked
 {