Add description for placement microversion 1.26
in the following APIs in the placement API reference.
* PUT /resource_providers/{uuid}/inventories
* PUT /resource_providers/{uuid}/inventories/{resource_class}
Up to microversion 1.25, the value of 'reserved' has to be less
than the value of 'total' in the request.
Starting from microversion 1.26, it has to be less than or
equal to the value of 'total'.
Change-Id: Id1857abf323c9776ab8fe957202848ff7ab3fbd2
Closes-Bug: #1783380
eec172a017