gobject: Document that classes/objects/interfaces are zero-filled

On initialisation, GObject guarantees to zero-fill
class/object/interface structures. Document this so people don’t spend
forever writing:
    my_object->priv->some_member = NULL;
    my_object->priv->some_other_member = NULL;

https://bugzilla.gnome.org/show_bug.cgi?id=729167

docs/reference/gobject/tut_howto.xml

@@ -147,7 +147,9 @@ GType maman_bar_get_type (void);
             macro with the G_DEFINE_TYPE_WITH_CODE() or the G_DEFINE_TYPE_EXTENDED()
             macros. The private structure is then defined in the .c file,
             and can be accessed using the <function>get_instance_private()</function>
-            function generated by the G_DEFINE_TYPE_* macros.
+            function generated by the G_DEFINE_TYPE_* macros. It is automatically
+            zero-filled on creation, so it is unnecessary to explicitly
+            initialize pointer members to NULL.
 <programlisting>
 struct _MamanBarPrivate
 {
@@ -333,7 +335,8 @@ maman_bar_init (MamanBar *self)
 {
   self->priv = maman_bar_get_instance_private (self);
 
-  /* initialize all public and private members to reasonable default values. */
+  /* initialize all public and private members to reasonable default values.
+   * They are all automatically initialized to 0 to begin with. */
 }
 </programlisting>
     </para>
@@ -574,7 +577,7 @@ maman_bar_do_action (MamanBar *self, /* parameters */)
         implementation for this class method in the object's
         <function>class_init</function> function: initialize the
         klass-&gt;do_action field to a pointer to the actual implementation.
-        By default, class method that are not inherited are initialized to
+        By default, class methods that are not inherited are initialized to
         NULL, and thus are to be considered "pure virtual".
 <programlisting>
 static void