locktorture: Add documentation

Just like Documentation/RCU/torture.txt, begin a document for the
locktorture module. This module is still pretty green, so I have
just added some specific sections to the doc (general desc, params,
usage, etc.). Further development should update the file.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
[ paulmck: Apply Randy Dunlap review comments. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>