[Qt] Improved copy: RBF checkbox, tooltip and confirmation screen

Opt-in RBF checkbox uses less technical jargon and emphasises
the fee bump functionality (at the expense of not mentioning
other uses of RBF).

The transaction confirmation screen uses copy consistent with this.

src/qt/sendcoinsdialog.cpp

@@ -345,7 +345,7 @@ void SendCoinsDialog::on_sendButton_clicked()
     if (ui->optInRBF->isChecked())
     {
         questionString.append("<hr /><span>");
-        questionString.append(tr("This transaction signals replaceability (optin-RBF)."));
+        questionString.append(tr("You can increase the fee later (signals Replace-By-Fee)."));
         questionString.append("</span>");
     }