docs/library/os.rst: Clarify littlefs requirements for block erase.
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@ -314,6 +314,12 @@ that the block device supports the extended interface.
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``ioctl(6, ...)`` must also be intercepted. The need for others is
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hardware dependent.
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Prior to any call to ``writeblocks(block, ...)`` littlefs issues
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``ioctl(6, block)``. This enables a device driver to erase the block
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prior to a write if the hardware requires it. Alternatively a driver
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might intercept ``ioctl(6, block)`` and return 0 (success). In this case
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the driver assumes responsibility for detecting the need for erasure.
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Unless otherwise stated ``ioctl(op, arg)`` can return ``None``.
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Consequently an implementation can ignore unused values of ``op``. Where
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``op`` is intercepted, the return value for operations 4 and 5 are as
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