Docs: use // for comments in examples

Without sgml mode, we can't escape /* as /* anymore,
so just switch to // for comments in examples.

gtk/gtktextview.c

@@ -7395,7 +7395,7 @@ gtk_text_view_reset_im_context (GtkTextView *text_view)
  *         return TRUE;
  *     }
  *
- *     /* Do some stuff */
+ *     // Do some stuff
  *
  *   return GTK_WIDGET_CLASS (gtk_foo_bar_parent_class)->key_press_event (widget, event);
  * }