Merge #12501: [qt] Improved "custom fee" explanation in tooltip

0bc095e [qt] Improved "custom fee" explanation in tooltip (Randolf Richardson)

Pull request description:

  Thanks to @dooglus for asking about this tooltip in Issue 12500.
  Reference:  https://www.github.com/bitcoin/bitcoin/issues/12500

  I would also appreciate it if someone can confirm that 1 kilobyte in this field indeed represents 1,000 bytes rather than 1,024 bytes (if it's supposed to be 1,024, then I'll gladly make the necessary changes to reflect this).

Tree-SHA512: da2fe0128411b5ef6f0a26382a80601efcf823c3f3591bdd83a7fe7e25777728e7eb89e2e8b175b991566e63838aca12d204792f981031b86e7b2ba28ca50021

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>