[docs/gcloud] add caveat about maximum value of GS_EXPIRATION in v4
@@ -204,6 +204,7 @@ Note: Default Google Compute Engine (GCE) Service accounts are
The ``GS_EXPIRATION`` value is handled by the underlying `Google library <https://googlecloudplatform.github.io/google-cloud-python/latest/storage/blobs.html#google.cloud.storage.blob.Blob.generate_signed_url>`_.
It supports `timedelta`, `datetime`, or `integer` seconds since epoch time.
+Note: The maximum value for this option is 7 days (604800 seconds) in version `v4` (See this `Github issue <https://github.com/googleapis/python-storage/issues/456#issuecomment-856884993>`_)
Usage
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