Since the DELETE /resource_providers/{u}/inventories and .../traits APIs
don't have a way to accept generation, they're not "threadsafe" in the
sense of multiple client threads managing traits/inventories for the
same provider. This change adds a note to the documentation for these
APIs to this effect, suggesting the use of PUT with empty
traits/inventories instead.
Change-Id: Icfd79cc1f5a912131845a22b4fe900147b19f934
Closes-Bug: #1746373
f3b0cf3975