Mention (unused) RP generation in POST /allocs/{c}

The schema accepts the (resource provider) 'generation' key in the
resource provider section of the allocations dictionary in the POST
/allocations/{consumer_uuid} request payload. As with PUT
/allocations/{consumer_uuid}, it is ignored. This was missing from the
API reference.

Change-Id: I2586875a60aba7a566f6c1b61ded133b8749031c

placement-api-ref/source/allocations.inc

@@ -41,6 +41,7 @@ Request
   - project_id: project_id_body
   - user_id: user_id_body
   - allocations: allocations_dict_empty
+  - generation: resource_provider_generation_optional
   - resources: resources
 
 Request example (microversions 1.28 - )

placement-api-ref/source/samples/allocations/manage-allocations-request-1.28.json

@@ -8,7 +8,8 @@
         "resources": {
           "VCPU": 2,
           "MEMORY_MB": 3
-        }
+        },
+        "generation": 4
       }
     }
   },
@@ -27,7 +28,8 @@
         "resources": {
           "VCPU": 4,
           "MEMORY_MB": 5
-        }
+        },
+        "generation": 12
       }
     }
   }