The current version of the spec (v1.2 WD 09) makes the "documentation" attribute of both the "operation" and "sensor" resources mandatory. This seems a little over the top. Although it seems likely that, in most cases, Providers will want to document the semantics of these resources, there are certainly cases in which the "name" and "description" attributes could provide enough information for Consumers to be able to figure out what the operation does or what the sensor measures.