docs/vm: move ksm and transhuge from "user" to "internals" section.

After the userspace interface description for KSM and THP was split to
Documentation/admin-guide/mm, the remaining parts belong to the section
describing MM internals.

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