PM / Hibernate: add section for resume options

Expand the existing documentation to explicitly list the options for
resuming a hibernation image, including the manual resume option which
can be used from the initrd or initramfs and the kernel init resume.

Signed-off-by: Sebastian Capella <sebastian.capella@linaro.org>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>