[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/forms/sendcoinsdialog.ui

@@ -1108,10 +1108,10 @@
            <item>
             <widget class="QCheckBox" name="optInRBF">
              <property name="text">
-              <string>Request Replace-By-Fee</string>
+              <string>Allow increasing fee</string>
              </property>
              <property name="toolTip">
-              <string>Indicates that the sender may wish to replace this transaction with a new one paying higher fees (prior to being confirmed).</string>
+              <string>This allows you to increase the fee later if the transaction takes a long time to confirm. This will also cause the recommended fee to be lower. ("Replace-By-Fee", BIP 125)</string>
              </property>
             </widget>
            </item>

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>");
     }