Documentation: update numastat explanation

Documentation: update numastat explanation

During recent patch discussion [1] it became apparent that the "other_node"
definition in the numastat documentation has always been different from actual
implementation. It was also noted that the stats can be innacurate on systems
with memoryless nodes.

This patch corrects the other_node definition (with minor tweaks to two more
definitions), adds a note about memoryless nodes and also two introductory
paragraphs to the numastat documentation.

[1] https://lore.kernel.org/linux-mm/20200504070304.127361-1-sandipan@linux.ibm.com/T/#u

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Sandipan Das <sandipan@linux.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Link: https://lore.kernel.org/r/20200507120217.12313-1-vbabka@suse.cz
Signed-off-by: Jonathan Corbet <corbet@lwn.net>