PM / hibernate: Clean up comments in snapshot.c

Many comments in kernel/power/snapshot.c do not follow the general
comment formatting rules.  They look odd, some of them are outdated
too, some are hard to parse and generally difficult to understand.

Clean them up to make them easier to comprehend.

No functional changes.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>