block: Correct documentation for BLOCK_WRITE_THRESHOLD

# means the device should be extended to avoid pausing for

# disk exhaustion.

# The event is one shot. Once triggered, it needs to be

# re-registered with another block-set-threshold command.

# re-registered with another block-set-write-threshold command.

#

# @node-name: graph node name on which the threshold was exceeded.

#