message: Clarify `leeway` meaning in `PaneGrid`
diff: @@ -211,6 +211,10 @@ impl<'a, Message, Renderer> PaneGrid<'a, Message, Renderer> {
  /// The `leeway` describes the amount of space around a split that can be
  /// used to grab it.
  ///
  + /// The grabbable area of a split will have a length of `spacing + leeway`,
  + /// properly centered. In other words, a length of
  + /// `(spacing + leeway) / 2.0` on either side of the split line.
  + ///
  /// [`PaneGrid`]: struct.PaneGrid.html
  pub fn on_resize<F>(mut self, leeway: u16, f: F) -> Self
  where
  