[Doc] Improve info window (#214)

* Documentation improvement

Slight tweak to avoid confusion with docs.

* [Doc] Improve info winfow doc

doc/overlay/info_window.md

@@ -114,9 +114,14 @@ $infoWindow->setOption('zIndex', 10);
 
 ## Append to a map/marker
 
-After building your info window, you need to add it to a map/marker with:
+After building your info window, you can add it to a map (the info window is draw at the info window coordinate):
 
 ``` php
 $map->getOverlayManager()->addInfoWindow($infoWindow);
+```
+
+Or you can attach it to a marker (the info window is draw on top of the marker):
+
+``` php
 $marker->setInfoWindow($infoWindow);
 ```