docs/uos: Make it clear that block device block_num param is an index.
@@ -193,14 +193,16 @@ used by a particular filesystem driver to store the data for its filesystem.
.. method:: readblocks(block_num, buf)
- Starting at *block_num*, read blocks from the device into *buf* (an array
- of bytes). The number of blocks to read is given by the length of *buf*,
+ Starting at the block given by the index *block_num*, read blocks from
+ the device into *buf* (an array of bytes).
+ The number of blocks to read is given by the length of *buf*,
which will be a multiple of the block size.
.. method:: writeblocks(block_num, buf)
- Starting at *block_num*, write blocks from *buf* (an array of bytes) to
- the device. The number of blocks to write is given by the length of *buf*,
+ Starting at the block given by the index *block_num*, write blocks from
+ *buf* (an array of bytes) to the device.
+ The number of blocks to write is given by the length of *buf*,
which will be a multiple of the block size.
.. method:: ioctl(op, arg)
