ACTION-2061 - Update the includenamespaceprefixes example

All,

This one is now done. Diff:


https://www.w3.org/community/xformsusers/wiki/index.php?title=XForms_2.0&diff=184&oldid=183

Text (search for "Author-optional attribute providing control over
namespace serialization"):


https://www.w3.org/community/xformsusers/wiki/XForms_2.0#The_submission_Element_2

I expanded the example to show what happens without the attribute, with an
empty attribute, and with an attribute containing a value.

-Erik