memory-hotplug.rst: add some details about locking internals

memory-hotplug.rst: add some details about locking internals

Let's document the magic a bit, especially why device_hotplug_lock is
required when adding/removing memory and how it all play together with
requests to online/offline memory from user space.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Reviewed-by: Pavel Tatashin <[email protected]>
Reviewed-by: Rashmica Gupta <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: John Allen <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kate Stewart <[email protected]>
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Mathieu Malaterre <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Michael Neuling <[email protected]>
Cc: Nathan Fontenot <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Philippe Ombredanne <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: YASUAKI ISHIMATSU <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>