Sync glance docs with metadefs api changes

 - Rename resource_type to resource_type_
   associations in namespace API input/output
 - Add created_at/updated_at in resource_
   type_associations block of namespace API
   input/output.
 - Fix minor nits

Closes-bug: 1367908

Change-Id: Ie74bfb36a7023abb346852f5b863f19d0d7bc4ff
Merge "Sync glance docs with metadefs api changes"