Note name of Mac sound plugin

doc/commandline.html

@@ -155,8 +155,9 @@ <h1>dopewars command line options</h1>
 <dt><b>-u <i>name</i></b>, <b>--plugin=<i>name</i></b></dt>
 <dd>Uses the named plugin for sound output. Valid options are "none" (for
 no sound) plus any name registered by plugins found on your system. (These
-are currently "winmm" for the Windows multimedia system plugin, and "esd"
-and "sdl" for Unix plugins using the ESound and SDL libraries.) If no such
+are currently "winmm" for the Windows multimedia system plugin, "cocoa" for the
+macOS audio plugin, and "esd" and "sdl" for Unix plugins using the ESound
+and SDL libraries respectively.) If no such
 option is given, the first valid sound plugin to be found is used.</dd>
 
 <dt><b>-h</b>, <b>--help</b></dt>