Add comment explaining why we set expires_utc to None

yt_dlp/cookies.py

@@ -347,6 +347,8 @@ def _process_chrome_cookie(decryptor, host_key, name, value, encrypted_value, pa
         if value is None:
             return is_encrypted, None
 
+    # In chrome, session cookies have has_expires and expires_utc set to 0
+    # In Python, cookies that do not expire have expires set to None
     if not has_expires:
         expires_utc = None