ksm: add zero_pages_sharing documentation

ksm: add zero_pages_sharing documentation

When enabling use_zero_pages, pages_sharing cannot represent how much
memory saved indeed.  zero_pages_sharing + pages_sharing does.  add the
description of zero_pages_sharing.

Link: https://lkml.kernel.org/r/202212300917284911971@zte.com.cn
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Cc: Xiaokai Ran <ran.xiaokai@zte.com.cn>
Cc: Yang Yang <yang.yang29@zte.com.cn>
Cc: Jiang Xuexin <jiang.xuexin@zte.com.cn>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>