Extend API documentation
    
    This includes:
    
    * Section about command/param info in usage guide
    * Section about metadata retrieval in usage guide
    * Guide about differences between CLI and API
    * Access control guide (management of roles, privileges and
      permissions).
    * Guide about API contexts
    * JSON-RPC usage guide and JSON-to-Python conversion
    * Notes about types in API Reference
    
    Signed-off-by: Antonio Torres <antorres@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>