libata: transport: cleanup documentation of sysfs interface

Clean-up the documentation of sysfs interfaces to be in the same format
as described in Documentation/ABI/README. This will be useful for
tracking changes in the ABI. Attributes are grouped by function (device,
link or port) and then by date added.

This patch also adds documentation for one attribute -
/sys/class/ata_port/ataX/port_no

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>