Update documentation and add attribute for schema_version element

[drothenberg]

With the new OVAL 5.11 versioning policy, the multiplicity of the generator's schema_version element was changed to "unbounded" to allow for accurate reporting of each platform extension's schema version. The new documentation suggested use of the xmlns attribute, which would be an incorrect application of this default XML attribute to provide metadata.

It would be more appropriate to add a new attribute to the schema_version element to differentiate between platform extensions referenced within an OVAL document.