docs: Remove XML-style comments from documentation strings

gtk-doc doesn’t support them any more since it was ported to Markdown,
so they end up appearing in the generated documentation, which isn’t
great.

Mostly, they were used to split up things invisibly, which we can do in
other ways.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: nobody

gobject/gtypemodule.h

@@ -93,7 +93,7 @@ struct _GTypeModuleClass
  * A convenience macro for dynamic type implementations, which declares a
  * class initialization function, an instance initialization function (see 
  * #GTypeInfo for information about these) and a static variable named 
- * @t_n<!-- -->_parent_class pointing to the parent class. Furthermore, 
+ * `t_n`_parent_class pointing to the parent class. Furthermore,
  * it defines a `*_get_type()` and a static `*_register_type()` functions
  * for use in your `module_init()`.
  *