Documentation: move oneshot trigger attributes documentation to ABI

Documentation of sysfs interface should be in ABI in the first place.
This moves relevant part of documentation and mentions where to look for
it.
Fix trivial typos whilst we are at it.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>