mm: better document PG_reserved

The usage of PG_reserved and how PG_reserved pages are to be treated is
buried deep down in different parts of the kernel.  Let's shine some
light onto these details by documenting current users and expected
behavior.

Especially, clarify on the "Some of them might not even exist" case.
These are physical memory gaps that will never be dumped as they are not
marked as IORESOURCE_SYSRAM.  PG_reserved does in general not hinder
anybody from dumping or swapping.  In some cases, these pages will not
be stored in the hibernation image.

Link: http://lkml.kernel.org/r/20190114125903.24845-10-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Anthony Yznaga <anthony.yznaga@oracle.com>
Cc: Miles Chen <miles.chen@mediatek.com>
Cc: <yi.z.zhang@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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