more doc

usage.md

@@ -16,8 +16,11 @@ if this filename is an empty string, *stdout* will be used.
 
 ## Use it
 
-- call genp_move(x, y)
+- real xa, ya, xb, yb
-- call genp_draw(x, y, c)
+- integer color
+- call genp_move(xa, ya)
+- call genp_draw(xa, ya, color)
+- call genp_line(xa, ya, xb, yb, color)
 
 ## Terminator