docs/protect-an-instance: add info about allowedServices in api keys

Author: wukko

docs/protect-an-instance.md

@@ -159,7 +159,8 @@ type KeyFileContents = Record<
         name?: string,
         limit?: number | "unlimited",
         ips?: (CIDRString | IPString)[],
-        userAgents?: string[]
+        userAgents?: string[],
+        allowedServices?: "all" | string[],
     }
 >;
 ```
@@ -179,6 +180,11 @@ where *`UUIDv4String`* is a stringified version of a UUIDv4 identifier.
     - when specified, requests with a `user-agent` that does not appear in this array will be rejected.
     - when omitted, any user agent can be specified to make requests with that API key.
 
+- **`allowedServices`** is an array of allowed services or `"all"`.
+    - when `"all"` is specified, the key will be able to access all supported services, even if they're globally disabled via `DISABLED_SERVICES`.
+    - when an array of services is specified, the key will be able to access only the services included in the array.
+    - when omitted, the key will use the global list of supported services.
+
 - if both `ips` and `userAgents` are set, the tokens will be limited by both parameters.
 - if cobalt detects any problem with your key file, it will be ignored and a warning will be printed to the console.