docs: sys/internal/counters/activity, explain deprecation for current…

…_billing_period (#28773)

per discussion
https://hashicorp.slack.com/archives/CPEPB6WRL/p1729786785076429
the reason this parameter is deprecated is because it's no longer required. make that clear in the docs.

website/content/api-docs/system/internal-counters.mdx

@@ -377,7 +377,7 @@ is unknown.
 - `current_billing_period` `(bool, optional)` - **DEPRECATED** Uses the builtin billing start
   timestamp as `start_time` and the current time as the `end_time`, returning a
   response with the current billing period information without having to
-  explicitly provide a start and end time.
+  explicitly provide a start and end time. This parameter is deprecated, as this option is now the default, so no parameter is needed to specify.
 
 
 ### Sample request
@@ -1367,4 +1367,4 @@ jdoe,jdoe,665a54bf-8652-c0c5-8121-40adf0d9786a,entity,false,wOsmr,ns1/a,auth_lda
 jdoe,jdoe,0640a8f0-b315-cc8a-c0c2-713f663774df,entity,false,oIiGy,ns1/b,auth_userpass_01a6ea85,auth/userpass/,userpass,2024-07-21T14:51:36Z,,new_york,east,,76a374a1-72fd-30ca-2455-f51dfeaa805e,secret-read,,,
 ```
 
-</CodeBlockConfig>
+</CodeBlockConfig>