The first child of 'attribute' can be an 'annotation' element.

Example:

<xs:attribute name="src" type="xs:anyURI">
<xs:annotation>
<xs:documentation xmlns="http://www.w3.org/1999/xhtml">
<p>This attribute is a link to a image object.  The value can be a relative URL.</p>
</xs:documentation>
</xs:annotation>
</xs:attribute>