reword the kspace_modify fftbench keyword docs to reflect the current state...

reword the kspace_modify fftbench keyword docs to reflect the current state (i.e. off by default) of code

doc/src/kspace_modify.txt

@@ -219,10 +219,10 @@ instead of using the virial equation. This option cannot be used to access
 individual components of the pressure tensor, to compute per-atom virial,
 or with suffix kspace/pair styles of MSM, like OMP or GPU.
 
-The {fftbench} keyword applies only to PPPM. It is on by default. If
-this option is turned off, LAMMPS will not take the time at the end
-of a run to give FFT benchmark timings, and will finish a few seconds
-faster than it would if this option were on.
+The {fftbench} keyword applies only to PPPM. It is off by default. If
+this option is turned on, LAMMPS will perform a short FFT benchmark
+computation and report its timings, and will thus finish a some seconds
+later than it would if this option were off.
 
 The {collective} keyword applies only to PPPM.  It is set to {no} by
 default, except on IBM BlueGene machines.  If this option is set to
@@ -306,7 +306,7 @@ parameters, see the "How-To"_Section_howto.html#howto_24 discussion.
 The option defaults are mesh = mesh/disp = 0 0 0, order = order/disp =
 5 (PPPM), order = 10 (MSM), minorder = 2, overlap = yes, force = -1.0,
 gewald = gewald/disp = 0.0, slab = 1.0, compute = yes, cutoff/adjust =
-yes (MSM), pressure/scalar = yes (MSM), fftbench = yes (PPPM), diff = ik
+yes (MSM), pressure/scalar = yes (MSM), fftbench = no (PPPM), diff = ik
 (PPPM), mix/disp = pair, force/disp/real = -1.0, force/disp/kspace = -1.0,
 split = 0, tol = 1.0e-6, and disp/auto = no. For pppm/intel, order =
 order/disp = 7.