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/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Anthony Yznaga <[email protected]>
Cc: Miles Chen <[email protected]>
Cc: <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Randy Dunlap <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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