docs/vm: transhuge: split userspace bits to admin-guide/mm/transhuge

Now that the administrative information for transparent huge pages is
nicely separated, move it to its own page under the admin guide.

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

Documentation/admin-guide/mm/index.rst

@@ -27,4 +27,5 @@ the Linux memory management.
    numa_memory_policy
    pagemap
    soft-dirty
+   transhuge
    userfaultfd