[trivial] fix mempool comment (outdated by BIP125)

Author: jnewbery

src/txmempool.h

@@ -328,14 +328,17 @@ struct TxMempoolInfo
 };
 
 /**
- * CTxMemPool stores valid-according-to-the-current-best-chain
- * transactions that may be included in the next block.
+ * CTxMemPool stores valid-according-to-the-current-best-chain transactions
+ * that may be included in the next block.
  *
- * Transactions are added when they are seen on the network
- * (or created by the local node), but not all transactions seen
- * are added to the pool: if a new transaction double-spends
- * an input of a transaction in the pool, it is dropped,
- * as are non-standard transactions.
+ * Transactions are added when they are seen on the network (or created by the
+ * local node), but not all transactions seen are added to the pool. For
+ * example, the following new transactions will not be added to the mempool:
+ * - a transaction which doesn't make the mimimum fee requirements.
+ * - a new transaction that double-spends an input of a transaction already in
+ * the pool where the new transaction does not meet the Replace-By-Fee
+ * requirements as defined in BIP 125.
+ * - a non-standard transaction.
  *
  * CTxMemPool::mapTx, and CTxMemPoolEntry bookkeeping:
  *