I get this invalid warning:

/home/treitter/checkout/gnome/folks/docs/gtk-doc/folks/ccomments/groupable.c:20: warning: unknown annotation "closure" in documentation for folks_groupable_change_group.

for the following chunk of documentation:

/**
 * folks_groupable_change_group
 * @self: the <link linkend="FolksGroupable"><type>FolksGroupable</type></link> instance
 * @group: (in): a freeform group identifier 
 * @is_member: (in): whether the contact should be a member of the group 
 * @_callback_: (scope async): callback to call when the request is satisfied
 * @_user_data_: (closure): the data to pass to @_callback_ function
 * 
 * Add or remove the contact from the specified group. 
 * 
 * <para>If <blockquote>is_member</blockquote> is <blockquote>true</blockquote>, the contact will be added to the <blockquote>group</blockquote>. If it is <blockquote>false</blockquote>, they will be removed from the <blockquote>group</blockquote>. </para>
 * 
 * <emphasis>See also</emphasis>: <link linkend="folks-groupable-change-group-finish"><function>folks_groupable_change_group_finish()</function></link>
 *
 * Since: 0.1.11
 */

=====================================

See the GObjectIntrospection annotation for (closure).