You can create annotations right after the xsd:schema element to document the entire schema.
<annotation id="ID"
{any attributes with non-schema namespace}>
Content: (appInfo | documentation)*
</annotation>
File: Schema.xsd
<xsd:schema xmlns:xsd="http://www.w3.org/2000/10/XMLSchema">
<xsd:annotation>
<xsd:documentation>
test
</xsd:documentation>
</xsd:annotation>
</xsd:schema>