Add "events" category to wxEvent-derived classes docs where missing

If a wxEvent-derived class documentation lacks "events" category,
the class will not appear in the Events class list.

The "events" category is still missing in several event classes
(e.g., palette events or wxRibbonToolBarEvent), but those lack
actual documentation aside from just methods declarations anyway.

interface/wx/html/htmlwin.h

@@ -559,7 +559,7 @@ wxEventType wxEVT_HTML_LINK_CLICKED;
     @endEventTable
 
     @library{wxhtml}
-    @category{html}
+    @category{html,events}
 */
 class wxHtmlLinkEvent : public wxCommandEvent
 {
@@ -592,7 +592,7 @@ public:
     @endEventTable
 
     @library{wxhtml}
-    @category{html}
+    @category{html,events}
 */
 class wxHtmlCellEvent : public wxCommandEvent
 {