docs/vm: ksm: reshuffle text between "sysfs" and "design" sections

The description of "max_page_sharing" sysfs attribute includes lots of
implementation details that more naturally belong in the "Design"
section.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>