Add documentation for the new snd_tinnyfilter.

doc/sopwith.cfg.5

@@ -40,6 +40,10 @@ indicating "false" and a non-zero value indicating "true".
 \fBvid_fullscreen\fR
 If 1, the game runs full screen rather than in a window. By default this is 0.
 .TP
+\fBsnd_tinnyfilter\fR
+If 1, a filter is applied to the sound output so that it sounds more like a
+tinny PC speaker that's inside a metal PC case. By default this is 1.
+.TP
 \fBconf_solidground\fR
 If 1, display the ground as a solid white block, reproducing the visual
 appearance of "Sopwith 1". If 0, the ground is displayed as a line, reproducing