ksm: add sysfs ABI Documentation

ksm: add sysfs ABI Documentation

Add sysfs documentation for Kernel Samepage Merging (KSM) including new
merge_across_nodes knob.

Signed-off-by: Petr Holasek <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>