dmenu.1: clarify monitor numbers are starting from 0 (first mon)

dmenu.1

@@ -52,7 +52,8 @@ dmenu matches menu items case insensitively.
 dmenu lists items vertically, with the given number of lines.
 .TP
 .BI \-m " monitor"
-dmenu is displayed on the monitor supplied.
+dmenu is displayed on the monitor number supplied. Monitor numbers are starting
+from 0.
 .TP
 .BI \-p " prompt"
 defines the prompt to be displayed to the left of the input field.