Document the conservation_mode attributes provided by the ideapad-laptop module.

Fixes: ade50296 ("platform/x86: ideapad-laptop: Expose conservation mode switch")
Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Link: https://lore.kernel.org/r/20210203215403.290792-30-pobrn@protonmail.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>