Rename FlushWalletDB -> CompactWalletDB, add function description

src/wallet/wallet.cpp

@@ -3765,7 +3765,7 @@ void CWallet::postInitProcess(CScheduler& scheduler)
 
     // Run a thread to flush wallet periodically
     if (!CWallet::fFlushScheduled.exchange(true)) {
-        scheduler.scheduleEvery(MaybeFlushWalletDB, 500);
+        scheduler.scheduleEvery(MaybeCompactWalletDB, 500);
     }
 }
 

src/wallet/walletdb.cpp

@@ -777,7 +777,7 @@ DBErrors CWalletDB::ZapWalletTx(vector<CWalletTx>& vWtx)
     return DB_LOAD_OK;
 }
 
-void MaybeFlushWalletDB()
+void MaybeCompactWalletDB()
 {
     static std::atomic<bool> fOneThread;
     if (fOneThread.exchange(true)) {

src/wallet/walletdb.h

@@ -193,6 +193,7 @@ private:
     void operator=(const CWalletDB&);
 };
 
-void MaybeFlushWalletDB();
+//! Compacts BDB state so that wallet.dat is self-contained (if there are changes)
+void MaybeCompactWalletDB();
 
 #endif // BITCOIN_WALLET_WALLETDB_H