Update ScanForWalletTransactions result comment

Suggested https://github.com/bitcoin/bitcoin/pull/14711#discussion_r252038666

src/wallet/wallet.cpp

@@ -1622,10 +1622,11 @@ int64_t CWallet::RescanFromTime(int64_t startTime, const WalletRescanReserver& r
  * @param[in] stop_block if not null, the scan will stop at this block instead
  *            of the chain tip
  *
- * @return ScanResult indicating success or failure of the scan. SUCCESS if
+ * @return ScanResult returning scan information and indicating success or
- * scan was successful. FAILURE if a complete rescan was not possible (due to
+ *         failure. Return status will be set to SUCCESS if scan was
- * pruning or corruption). USER_ABORT if the rescan was aborted before it
+ *         successful. FAILURE if a complete rescan was not possible (due to
- * could complete.
+ *         pruning or corruption). USER_ABORT if the rescan was aborted before
+ *         it could complete.
  *
  * @pre Caller needs to make sure start_block (and the optional stop_block) are on
  * the main chain after to the addition of any new keys you want to detect