[DOC] Update `set_trace_func` document

- Clarify the class of event parameters
- Represent event names as strings
- Update the example to show the above

vm_trace.c

@@ -521,25 +521,25 @@ static void call_trace_func(rb_event_flag_t, VALUE data, VALUE self, ID id, VALU
  *
  * _proc_ takes up to six parameters:
  *
- * * an event name
- * * a filename
+ * * an event name string
+ * * a filename string
  * * a line number
- * * an object id
- * * a binding
- * * the name of a class
+ * * a method name symbol, or nil
+ * * a binding, or nil
+ * * the class, module, or nil
  *
  * _proc_ is invoked whenever an event occurs.
  *
  * Events are:
  *
- * +c-call+:: call a C-language routine
- * +c-return+:: return from a C-language routine
- * +call+:: call a Ruby method
- * +class+:: start a class or module definition
- * +end+:: finish a class or module definition
- * +line+:: execute code on a new line
- * +raise+:: raise an exception
- * +return+:: return from a Ruby method
+ * <code>"c-call"</code>:: call a C-language routine
+ * <code>"c-return"</code>:: return from a C-language routine
+ * <code>"call"</code>:: call a Ruby method
+ * <code>"class"</code>:: start a class or module definition
+ * <code>"end"</code>:: finish a class or module definition
+ * <code>"line"</code>:: execute code on a new line
+ * <code>"raise"</code>:: raise an exception
+ * <code>"return"</code>:: return from a Ruby method
  *
  * Tracing is disabled within the context of _proc_.
  *
@@ -550,24 +550,25 @@ static void call_trace_func(rb_event_flag_t, VALUE data, VALUE self, ID id, VALU
  *     end
  *   end
  *
- *   set_trace_func proc { |event, file, line, id, binding, classname|
- *      printf "%8s %s:%-2d %10s %8s\n", event, file, line, id, classname
+ *   set_trace_func proc { |event, file, line, id, binding, class_or_module|
+ *     printf "%8s %s:%-2d %16p %14p\n", event, file, line, id, class_or_module
  *   }
  *   t = Test.new
  *   t.test
  *
  * Produces:
  *
- *       line prog.rb:11               false
- *     c-call prog.rb:11        new    Class
- *     c-call prog.rb:11 initialize   Object
- *   c-return prog.rb:11 initialize   Object
- *   c-return prog.rb:11        new    Class
- *       line prog.rb:12               false
- *       call prog.rb:2        test     Test
- *       line prog.rb:3        test     Test
- *       line prog.rb:4        test     Test
- *     return prog.rb:4        test     Test
+ *   c-return prog.rb:8   :set_trace_func         Kernel
+ *       line prog.rb:11              nil            nil
+ *     c-call prog.rb:11             :new          Class
+ *     c-call prog.rb:11      :initialize    BasicObject
+ *   c-return prog.rb:11      :initialize    BasicObject
+ *   c-return prog.rb:11             :new          Class
+ *       line prog.rb:12              nil            nil
+ *       call prog.rb:2             :test           Test
+ *       line prog.rb:3             :test           Test
+ *       line prog.rb:4             :test           Test
+ *     return prog.rb:5             :test           Test
  *
  * Note that for +c-call+ and +c-return+ events, the binding returned is the
  * binding of the nearest Ruby method calling the C method, since C methods