ata: Add documentation for hard disk shock protection interface (v3)

Put some information (and pointers to more) into the kernel's doc tree,
describing briefly the interface to the kernel's disk head unloading
facility. Information about how to set up a complete shock protection
system under GNU/Linux can be found on the web and is referenced
accordingly.

v3:
Here is some final polish including various spelling corrections
pointed out by Grant Grundler and Peter Moulder. Also, I have added some
information about the timing constraints related to disk head parking.
The patch looks more impressive than it really is and I think it would
be alright just to incorporate it into the original patch so as not to
clutter up the git log.

Signed-off-by: Elias Oltmanns <eo@nebensachen.de>
Cc: Jeff Garzik <jeff@garzik.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Tejun Heo <htejun@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>