rcutorture: Update rcutorture.fwd_progress help text

This commit updates the rcutorture.fwd_progress help text to say that
it is the number of forward-progress kthreads to spawn rather than the
old enable/disable functionality.  While in the area, make the list of
torture-test parameters easier to read by taking advantage of 100 columns.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>