Documentation/ABI: Add new attribute for mlxreg-io sysfs interfaces

Add documentation for the new attribute fan_dir (fan direction for
systems of types MQM87xx, MSN34xx, MSN37xx.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
[dvhart: minor gramatical correction]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>