Update docstring of bytes_per_pack.
Update docstring of bytes_per_pack.

Logic for bytes_per_pack hint is implemented in collective_ops.py, used by MirroredExtended and also CollectiveAllReduceExtended(multiwork mirrored strategy). Thus both strategy respects the hint.

PiperOrigin-RevId: 413526767
Change-Id: I8f17aaf7735329b8c5d9e4e075bd638793dc575f

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