Remove documentation references to region parameter for Stats Usage endpoints
The documentation for Stats Usage endpoints (/stats/usage, /stats/usage_by_month, and /stats/usage_by_service) made reference to an unsupported region query parameter. The documentation has been updated to remove references to this parameter. No changes were made to the API itself.
Prior change: Rust SDK 0.8.1
Following change: AssemblyScript SDK 0.4.2
