You can use XHTML inside your schema to describe the elements/types/etc.

For example:

<xs:annotation>
<xs:documentation xmlns="http://www.w3.org/1999/xhtml">
<p>This schema is use to encode...</p>
</xs:documentation>
</xs:annotation>

You might want to define a prefix ('h' in the example below) for XHTML so you don't have to repeat the default namespace declaration:

<xs:annotation>
<xs:documentation>
<h:p>This schema is use to encode...</h:p>
</xs:documentation>
</xs:annotation>

Keep in mind that the default namespace is used to resolve unprefixed QNames in XML Schema.