msi-laptop: Add missing ABI documentation

Add ABI documentation for all sysfs files exposed by msi-laptop driver.

Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>