doc: Clarify that feerates are per virtual size

src/qt/forms/sendcoinsdialog.ui

@@ -850,7 +850,7 @@
                    <property name="toolTip">
                     <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>
+Note:  Since the fee is calculated on a per-byte basis, a fee rate of "100 satoshis per kvB" for a transaction size of 500 virtual bytes (half of 1 kvB) would ultimately yield a fee of only 50 satoshis.</string>
                    </property>
                    <property name="text">
                     <string>per kilobyte</string>