thp: documentation: 'transparent_hugepage=' can also be specified on cmdline

The behavior of THP can either be toggled through sysfs in runtime or
using a kernel cmdline parameter 'transparent_hugepage='.  Document the
latter in kernel-parameters.txt

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>