Document that #m/mousectl's accelerate command takes an optional scaling factor.

Reference: /n/sources/patch/applied/man-3-mouse-accel
Date: Sat Jan  2 00:16:41 CET 2010
Signed-off-by: lyndon@orthanc.ca

--- /sys/man/3/mouse	Sat Jan  2 00:15:08 2010
+++ /sys/man/3/mouse	Sat Jan  2 00:15:06 2010
@@ -100,8 +100,10 @@
 followed by a write of
 .BR intellimouse .
 .TP
-.B accelerated
+.B accelerated [\fIn\fP]
 turns on mouse acceleration.
+.I n
+specifies an optional acceleration factor.
 .TP
 .B linear
 turns off mouse acceleration