Merge "Enhance simple usage API documentation"

Author: Zuul

api-ref/source/os-simple-tenant-usage.inc

@@ -8,6 +8,18 @@ Reports usage statistics of compute and storage resources periodically
 for an individual tenant or all tenants. The usage statistics will include
 all instances' CPU, memory and local disk during a specific period.
 
+.. warning::
+
+   The `os-simple-tenant-usage` will report usage statistics based on the latest
+   flavor that is configured in the virtual machine (VM), and ignoring stop,
+   pause, and other events that might have happened with the VM. Therefore, it
+   uses the time the VM existed in the cloud environment to execute the usage
+   accounting.
+
+   More information can be found at
+   http://eavesdrop.openstack.org/meetings/nova/2020/nova.2020-12-03-16.00.log.txt,
+   and https://review.opendev.org/c/openstack/nova/+/711113
+
 Microversion 2.40 added pagination (and ``next`` links) to the usage
 statistics via optional ``limit`` and ``marker`` query parameters. If
 ``limit`` isn't provided, the configurable ``max_limit`` will be used which