SchemaType needs annotation.

SchemaType should have an annotation that describes its use of open 
content for specified behavior. The annotation can be brief, but it 
should get across the idea that an <spml:schema> may refer to an 
<xsd:schema> document or may contain an <xsd:schema> document as open 
content.

This instance deserves annotation because we rely on open content for 
behavior that is specified in the protocol (rather than simply to allow 
for behavior that is unspecified). 

I'm also guided in this by the questions and issues raised by Kent 
Spaulding, the developer working on our implementation for the Interop.  
Things that are unclear to him are likely to be unclear to others less 
familiar with SPML2.