[qt] Improved "custom fee" explanation in tooltip

Author: randolf

src/qt/forms/sendcoinsdialog.ui

@@ -848,7 +848,9 @@
                  <item>
                   <widget class="QLabel" name="labelCustomPerKilobyte">
                    <property name="toolTip">
-                    <string>If the custom fee is set to 1000 satoshis and the transaction is only 250 bytes, then &quot;per kilobyte&quot; only pays 250 satoshis in fee, while &quot;total at least&quot; pays 1000 satoshis. For transactions bigger than a kilobyte both pay by kilobyte.</string>
+                    <string>Specify a custom fee per kB (1,000 bytes) of the transaction's virtual size.
+
+Note:  Since the fee is calculated on a per-byte basis, a fee of "100 satoshis per kB" for a transaction size of 500 bytes (half of 1 kB) would ultimately yield a fee of only 50 satoshis.</string>
                    </property>
                    <property name="text">
                     <string>per kilobyte</string>