docs: block: fix discard_max_bytes references

docs: block: fix discard_max_bytes references

When discard_max_hw_bytes was introduced, in commit 0034af036554
("block: make /sys/block/<dev>/queue/discard_max_bytes writeable"),
the discard_max_bytes documentation section was renamed to
discard_max_hw_bytes, but the references it contains to
discard_max_bytes weren't updated.

This patch refers to discard_max_hw_bytes instead. It also reflows the
paragraph so that sentences end lines.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20210910105142.2116749-1-steve@sk2.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>