Correct a spelling mistake in the Sixaxis HID descriptor comment.

Add an explanation as to why the Sixaxis has so many analog axes and why
some of them are seen as multi-touch axes.
Signed-off-by: NFrank Praznik <frank.praznik@oh.rr.com>
Acked-by: NPavel Machek <pavel@ucw.cz>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>