PM / hibernate: clarify resume documentation

it was not the whole truth that kernel mode cannot be used with swap on LVM

Signed-off-by: Uwe Geuder <linuxkernel2015-ugeuder@snkmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>