Merge bitcoin#23341: RPC: Better safety with newkeypool command and wallet backups

a2a9231 rpc: Add warning to user about newkeypool command (Samuel Dobson)

Pull request description:

  This PR prevents `newkeypool` from being run on non-HD wallets, because this would require a new backup every time, so it isn't very safe.

  David Harding also suggested [here](bitcoin#23093 (comment)) that the RPC help text should include a warning to the users about the interaction between newkeypool.

ACKs for top commit:
  achow101:
    ACK a2a9231

Tree-SHA512: 0aa497900f1d179764bce13ffce0bb081ba2ca354492bf2e04b21d0212e960b3ed13a386fbf65602b6b50461f4056a0285752ef707d312da28e82449cd8ea048

Author: achow101

src/wallet/rpcwallet.cpp

@@ -1789,7 +1789,12 @@ static RPCHelpMan keypoolrefill()
 static RPCHelpMan newkeypool()
 {
     return RPCHelpMan{"newkeypool",
-                "\nEntirely clears and refills the keypool."+
+                "\nEntirely clears and refills the keypool.\n"
+                "WARNING: On non-HD wallets, this will require a new backup immediately, to include the new keys.\n"
+                "When restoring a backup of an HD wallet created before the newkeypool command is run, funds received to\n"
+                "new addresses may not appear automatically. They have not been lost, but the wallet may not find them.\n"
+                "This can be fixed by running the newkeypool command on the backup and then rescanning, so the wallet\n"
+                "re-generates the required keys." +
             HELP_REQUIRING_PASSPHRASE,
                 {},
                 RPCResult{RPCResult::Type::NONE, "", ""},