docs: Replace <function> XML tag with back ticks

https://bugzilla.gnome.org/show_bug.cgi?id=731050

gobject/gtype.h

@@ -1076,8 +1076,7 @@ struct _GInterfaceInfo
  * @value_init: Default initialize @values contents by poking values
  *  directly into the value->data array. The data array of
  *  the #GValue passed into this function was zero-filled
- *  with <function>memset()</function>, so no care has to
- *  be taken to free any
+ *  with `memset()`, so no care has to be taken to free any
  *  old contents. E.g. for the implementation of a string
  *  value that may never be %NULL, the implementation might
  *  look like:

gobject/gtypemodule.h

@@ -92,9 +92,9 @@ struct _GTypeModuleClass
  * 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, 
- * it defines a <function>*_get_type()</function> and a static 
- * <function>*_register_type()</function> function for use in your
- * <function>module_init()</function>.
+ * it defines a `*_get_type()` and a static `*_register_type()` functions
+ * for use in your `module_init()`.
+ *
  * See G_DEFINE_DYNAMIC_TYPE_EXTENDED() for an example.
  * 
  * Since: 2.14