message: Fix reserve balance tooltip
diff: <item>
  <widget class="BitcoinAmountField" name="reserveBalance">
  <property name="toolTip">
  - <string>(0 = auto, &lt;0 = leave that many cores free)</string>
  + <string>Reserve amount that will not be used for staking</string>
  </property>
  </widget>
  </item>
  