The first child of 'complexType' or 'simpleType' can be an 'annotation' element.

Example:

<xs:complexType name="XHTMLBlockContents">
<xs:annotation>
<xs:documentation xmlns="http://www.w3.org/1999/xhtml">
<p>This type represents a container of XHTML block elements.</p>
</xs:documentation>
</xs:annotation>
...
</xs:complexType>