Issue #358 cont'd: Fl_Cairo_Window coord system docs

Elaborated on Fl_Cairo_Window's use of FLTK style coordinates,
    and how this differs from cairo's default native normalized
    coordinate system, and shows how to switch from one to the other.

    Also, small comment fix to the cairo example regarding the "X" color.

examples/cairo-draw-x.cxx

@@ -24,7 +24,7 @@ static void my_cairo_draw_cb(Fl_Cairo_Window *window, cairo_t *cr) {
     const double xmax = (window->w() - 1);
     const double ymax = (window->h() - 1);
 
-    // Draw green "X"
+    // Draw orange "X"
     //     Draws an X to four corners of resizable window.
     //     See Fl_Cairo_Window docs for more info.
     //