[trivial] Add comment documenting CWalletTx::mapValue

src/wallet/wallet.h

@@ -248,6 +248,31 @@ class CWalletTx : public CMerkleTx
     const CWallet* pwallet;
 
 public:
+    /**
+     * Key/value map with information about the transaction.
+     *
+     * The following keys can be read and written through the map and are
+     * serialized in the wallet database:
+     *
+     *     "comment", "to"   - comment strings provided to sendtoaddress,
+     *                         sendfrom, sendmany wallet RPCs
+     *     "from", "message" - obsolete fields that could be set in UI prior to
+     *                         2011 (removed in commit 4d9b223)
+     *
+     * The following keys are serialized in the wallet database, but shouldn't
+     * be read or written through the map (they will be temporarily added and
+     * removed from the map during serialization):
+     *
+     *     "fromaccount"     - serialized strFromAccount value
+     *     "n"               - serialized nOrderPos value
+     *     "timesmart"       - serialized nTimeSmart value
+     *     "spent"           - serialized vfSpent value that existed prior to
+     *                         2014 (removed in commit 93a18a3)
+     *
+     * A mapValue.erase("version") statement also appears in the code,
+     * originating from commit 865c3a2 in 2010, but it does not appear that any
+     * "version" entries were actually ever created by any version of bitcoin.
+     */
     mapValue_t mapValue;
     std::vector<std::pair<std::string, std::string> > vOrderForm;
     unsigned int fTimeReceivedIsTxTime;