docking: say that the drag bar's menu opens on a right click

A left click on the drag bar focuses it and opens nothing, as designed;
the Hit::DragBar docs said otherwise.

crates/ui/src/docking/hits.rs

@@ -14,7 +14,8 @@ use super::{DockArea, Floating, Layout, Node, render::RESIZE_HANDLES};
 pub enum Hit<P> {
     /// A tab: click activates, drag moves the panel (S1).
     Tab(P),
-    /// A group's drag bar: click opens the group menu, drag moves the group (S2).
+    /// A group's drag bar: drag moves the group; a right click, Enter or Space opens the group
+    /// menu (S2).
     DragBar(P),
     /// The top border of a group's strip: drag moves the group (S3).
     Border(P),

crates/ui/src/docking/render/group.rs

@@ -152,8 +152,9 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
         ui.painter().rect_stroke(rect, 0.0, self.style.border, StrokeKind::Inside);
     }
 
-    /// The drag bar (`drag-bar`: click, double-click, menu and drag) or the top border band
-    /// (`drag-border`: drag only). Both move the whole group.
+    /// The drag bar (`drag-bar`: drag, double-click, and the group menu on a right click, Enter
+    /// or Space) or the top border band (`drag-border`: drag only). Both move the whole group;
+    /// a left click on the bar only focuses it.
     fn group_handle(&mut self, ui: &mut Ui, name: &'static str, hit: Rect, owner: &TabGroup<'_, P>, movable: bool) {
         if !crate::docking::targets::usable(hit) {
             return;

docs/ui-docking.md

@@ -180,6 +180,8 @@ A flyout fits its content the same way, without the title bar.
 - Drop previews show the destination: an outline for tabs, a line for a new group, column or dock,
   a filled half for a split. Escape or losing window focus cancels a drag.
 - Double-click a tab or drag bar to minimize a group to its strip.
+- Right-click a tab for its menu, or a drag bar for the group menu; Enter or Space on a focused
+  drag bar opens it too. A left click on the drag bar only focuses it.
 - In the Frame profile, the mouse wheel over a tab strip selects the next or previous tab, and a
   middle click on a closable panel's tab closes it. `DockStyle::wheel_selects_tabs` and
   `middle_click_closes_tabs` turn either on or off in any profile.