doc tweak

regions.scad

@@ -1192,6 +1192,8 @@ function _list_three(a,b,c) =
 //   When called as a function and given a list of regions or 2D polygons,
 //   returns the union of all given regions and polygons.  Result is a single region.
 //   When called as the built-in module, makes the union of the given children.
+//   This function is **much** slower than the native union module acting on geometry,
+//   so you should only use it when you need a point list for further processing.  
 // Arguments:
 //   regions = List of regions to union.
 // Example(2D):
@@ -1227,6 +1229,8 @@ function union(regions=[],b=undef,c=undef,eps=EPSILON) =
 //   takes the first region or polygon and differences away all other regions/polygons from it.  The resulting
 //   region is returned.
 //   When called as the built-in module, makes the set difference of the given children.
+//   This function is **much** slower than the native difference module acting on geometry,
+//   so you should only use it when you need a point list for further processing.  
 // Arguments:
 //   regions = List of regions or polygons to difference.
 // Example(2D):
@@ -1299,6 +1303,8 @@ function intersection(regions=[],b=undef,c=undef,eps=EPSILON) =
 //   When called as a module, performs a Boolean exclusive-or of up to 10 children.  Note that when
 //   the input regions cross each other the exclusive-or operator will produce shapes that
 //   meet at corners (non-simple regions), which do not render in CGAL.  
+//   This function is **much** slower than the native intersection module acting on geometry,
+//   so you should only use it when you need a point list for further processing.  
 // Arguments:
 //   regions = List of regions or polygons to exclusive_or
 // Example(2D): As Function.  A linear_sweep of this shape fails to render in CGAL.