libata: update documentation for sysfs interfaces

Dcoumentation has been added by parsing through git commit history and
reading code. This might be useful for scripting and tracking changes in
the ABI.

I do not have complete descriptions for the following 3 attributes; they
have been annotated with the comment [to be documented] -

	/sys/class/scsi_host/hostX/ahci_port_cmd
	/sys/class/scsi_host/hostX/ahci_host_caps
	/sys/class/scsi_host/hostX/ahci_host_cap2

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