change parameter descriptions

Author: rculpepper

content/vault/v1.21.x (rc)/content/api-docs/system/billing-pki-certificate-count.mdx

@@ -13,18 +13,21 @@ number of PKI certificates issued by Vault. for a cluster.
 ## PKI Certificate Counts
 
 The Certificate Counts endpoint returns the number of PKI certificates issued by Vault, or an error, for each month of the given time range.
+By default, the endpoint returns data for the current billing period.
 
 | Method | Path                        |
 | :----- | :-------------------------- |
 | `GET`  | `/sys/billing/certificates` |
 
 ### Request parameters
 
-- `start_time` `(string: <required>)` - The first month for which to return certificate counts.
-The time must be a timestamp in RFC 3339 format (e.g., `2025-01-01T00:00:00Z` for January 2025).
+- `start_time` `(string: "")` - The year and month indicating the first month of the
+query range in format `YYYY-MM`. The `start_time` is required if an `end_time` is provided.
 
-- `end_time` `(string: <required>)` - The last month for which to return certificate counts. The `end_time` must not be before the `start_time`.
-The time must be a timestamp in RFC 3339 format (e.g., `2025-01-01T00:00:00Z` for January 2025).
+
+- `end_time` `(string: "")` - The year and month indicating the last month of the
+query range in format `YYYY-MM`. The `end_time` is required if a `start_time`
+is provided.
 
 ### Sample payload