Document optional RB/RW/RL argument

src/asm/rgbasm.5

@@ -851,6 +851,12 @@ There are five commands in the RS group of commands:
 (In practice, this one cannot be used due to a bug).
 .El
 .Pp
+If the argument to
+.Ic RB , RW ,
+or
+.Ic RL
+is omitted, it's assumed to be 1.
+.Pp
 Note that colons
 .Ql \&:
 following the name are not allowed.