Expand and clarify popover XML documentation

Significantly improve and expand XML documentation for IPopover, PopoverBaseImpl, and PopoverMenu. Add detailed remarks on popover lifecycle, registration requirements, focus, input, and mouse handling. Provide usage examples, parameter and exception documentation, and clarify method/property summaries. Warn about the need to register popovers before showing. No functional code changes except for minor summary tweaks for clarity.

Examples/UICatalog/Scenarios/Menus.cs

@@ -347,6 +347,8 @@ public class Menus : Scenario
             };
 
             ContextMenu.EnableForDesign (ref host);
+            Application.Popover.Register (ContextMenu);
+
             ContextMenu.Visible = false;
 
             // Demo of PopoverMenu as a context menu