docs: explain unused audience in GCP ADC strategy

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: cloudsmith-iduffy

cloudsmith_cli/core/credentials/oidc/detectors/gcp.py

@@ -127,6 +127,9 @@ def _token_from_service_account_file(self, audience: str) -> str | None:
             return None
 
     def _token_from_adc(self, audience: str) -> str | None:
+        # audience is unused here: ADC user credentials mint an id_token whose
+        # aud is gcloud's OAuth client id, not a custom audience. The signature
+        # stays uniform so get_token can call every strategy as strategy(audience).
         try:
             import google.auth  # pylint: disable=import-error
             from google.auth.transport.requests import (  # pylint: disable=import-error