ksm: add profit monitoring documentation

Add the description of KSM profit and how to determine it separately in
system-wide range and inner a single process.

Link: https://lkml.kernel.org/r/20220830144003.299870-1-xu.xin16@zte.com.cn


Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Izik Eidus <izik.eidus@ravellosystems.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>