Update 702-math-polyext.markdown

Author: Joe7M

_build/reference/702-math-polyext.markdown

@@ -1,14 +1,19 @@
 # POLYEXT
 
-> POLYEXT poly(), BYREF xmin, BYREF ymin, BYREF xmax, BYREF ymax
+> POLYEXT poly, BYREF xmin, BYREF ymin, BYREF xmax, BYREF ymax
 
-Returns the polyline's extents.
+Returns the extents of the polyline `poly`. `xmin` and `xmax` are extents in x-direction and `ymin` and `ymax` are the extents in y-direction. 
+
+See DRAWPOLY on how to define a polyline.
+
+### Example
 
 ```
 polygon = [ [30,50], [100,70], [120,90], [50,100], [30,50]]
 
 polyext(polygon, PolyXmin, PolyYmin, PolyXmax, PolyYmax)
 
+' Draw polygon and a rectangle for indicating the extents
 drawpoly polygon color 12
 rect PolyXmin, PolyYmin, PolyXmax, PolyYmax  color 5