gobject: Use _ prefixing in private headers over G_GNUC_INTERNAL in public headers

This helps out gtk-doc and g-ir-scanner, and also makes much clearer
what's private versus public.

https://bugzilla.gnome.org/show_bug.cgi?id=651745

docs/reference/gobject/gobject-sections.txt

@@ -162,16 +162,6 @@ G_TYPE_RESERVED_USER_FIRST
 
 <SUBSECTION Private>
 GOBJECT_VAR
-g_value_c_init
-g_value_types_init
-g_enum_types_init
-g_param_type_init
-g_boxed_type_init
-g_object_type_init
-g_param_spec_types_init
-g_value_transforms_init
-g_signal_init
-g_gtype_get_type
 </SECTION>
 
 <SECTION>

docs/reference/gobject/tut_gtype.xml

@@ -337,7 +337,7 @@ G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT)
         <para>
           A lot of types are not instantiable by the type system and do not have
           a class. Most of these types are fundamental trivial types such as <emphasis>gchar</emphasis>, 
-          registered in <function>g_value_types_init</function> (in <filename>gvaluetypes.c</filename>).
+          registered in <function>_g_value_types_init</function> (in <filename>gvaluetypes.c</filename>).
         </para>
 
         <para>