During code review of the unified limits implementation, reviewers
requested detail around the enforcement of "API limits" (limits that
are enforced only from the content of an API request). This adds a
paragraph to explain how they are enforced using the oslo.limit APIs.
A note about the [workaround]unified_limits_count_pcpu_as_vcpu config
option is also added to explain how operators can opt in to legacy
VCPU + PCPU quota usage counting behavior.
Related to bp unified-limits-nova
Change-Id: I41e7b8552763850a94374a6f31f1be72e7bf1626
385d6d25a2