Documentation/kernel-parameters.txt: document libata's ignore_hpa option

By default the kernel honors the HPA (host protected area) of hard
drives.  Using libata's ignore_hpa module option it's possible to
change this behaviour.

Document usage and options of libata.ignore_hpa in
Documentation/kernel-parameters.txt.

Signed-off-by: Michael Prokop <mika@grml.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>