docs/library/uos.rst: Improve block devices section, and ioctl ret vals.

docs/library/uos.rst

@@ -218,11 +218,19 @@ represented by VFS classes.
 Block devices
 -------------
 
-A block device is an object which implements the block protocol, which is a set
-of methods described below by the :class:`AbstractBlockDev` class.  A concrete
-implementation of this class will usually allow access to the memory-like
-functionality a piece of hardware (like flash memory).  A block device can be
-used by a particular filesystem driver to store the data for its filesystem.
+A block device is an object which implements the block protocol. This enables a
+device to support MicroPython filesystems. The physical hardware is represented
+by a user defined class. The :class:`AbstractBlockDev` class is a template for
+the design of such a class: MicroPython does not actually provide that class,
+but an actual block device class must implement the methods described below.
+
+A concrete implementation of this class will usually allow access to the
+memory-like functionality of a piece of hardware (like flash memory). A block
+device can be formatted to any supported filesystem and mounted using ``uos``
+methods.
+
+See :ref:`filesystem` for example implementations of block devices using the
+two variants of the block protocol described below.
 
 .. _block-device-interface:
 
@@ -294,5 +302,12 @@ that the block device supports the extended interface.
             (*arg* is unused)
           - 6 -- erase a block, *arg* is the block number to erase
 
-See :ref:`filesystem` for example implementations of block devices using both
-protocols.
+       As a minimum ``ioctl(4, ...)`` must be intercepted; for littlefs
+       ``ioctl(6, ...)`` must also be intercepted. The need for others is
+       hardware dependent.
+
+       Unless otherwise stated ``ioctl(op, arg)`` can return ``None``.
+       Consequently an implementation can ignore unused values of ``op``. Where
+       ``op`` is intercepted, the return value for operations 4 and 5 are as
+       detailed above. Other operations should return 0 on success and non-zero
+       for failure, with the value returned being an ``OSError`` errno code.