Add documentation describing CWallet::nTimeSmart.

Most of the text comes from the 2012 Luke Dashjr <luke-jr+git@utopios.org>
c3f95ef commit message.

src/wallet/wallet.cpp

@@ -3454,6 +3454,27 @@ void CWallet::GetKeyBirthTimes(std::map<CTxDestination, int64_t> &mapKeyBirth) c
         mapKeyBirth[it->first] = it->second->GetBlockTime() - 7200; // block times can be 2h off
 }
 
+/**
+ * Compute smart timestamp for a transaction being added to the wallet.
+ *
+ * Logic:
+ * - If sending a transaction, assign its timestamp to the current time.
+ * - If receiving a transaction outside a block, assign its timestamp to the
+ *   current time.
+ * - If receiving a block with a future timestamp, assign all its (not already
+ *   known) transactions' timestamps to the current time.
+ * - If receiving a block with a past timestamp, before the most recent known
+ *   transaction (that we care about), assign all its (not already known)
+ *   transactions' timestamps to the same timestamp as that most-recent-known
+ *   transaction.
+ * - If receiving a block with a past timestamp, but after the most recent known
+ *   transaction, assign all its (not already known) transactions' timestamps to
+ *   the block time.
+ *
+ * For more information see CWalletTx::nTimeSmart,
+ * https://bitcointalk.org/?topic=54527, or
+ * https://github.com/bitcoin/bitcoin/pull/1393.
+ */
 unsigned int CWallet::ComputeTimeSmart(const CWalletTx& wtx) const
 {
     unsigned int nTimeSmart = wtx.nTimeReceived;

src/wallet/wallet.h

@@ -260,6 +260,15 @@ public:
     std::vector<std::pair<std::string, std::string> > vOrderForm;
     unsigned int fTimeReceivedIsTxTime;
     unsigned int nTimeReceived; //!< time received by this node
+    /**
+     * Stable timestamp that never changes, and reflects the order a transaction
+     * was added to the wallet. Timestamp is based on the block time for a
+     * transaction added as part of a block, or else the time when the
+     * transaction was received if it wasn't part of a block, with the timestamp
+     * adjusted in both cases so timestamp order matches the order transactions
+     * were added to the wallet. More details can be found in
+     * CWallet::ComputeTimeSmart().
+     */
     unsigned int nTimeSmart;
     /**
      * From me flag is set to 1 for transactions that were created by the wallet