mention ArrayAccess restriction (closes #4632)

std/StdTypes.hx

@@ -115,5 +115,9 @@ typedef Iterable<T> = {
 /**
 	ArrayAccess is used to indicate a class that can be accessed using brackets.
 	The type parameter represents the type of the elements stored.
+
+	This interface should be used for externs only. Haxe does not support custom
+	array access on classes. However, array access can be implemented for
+	abstract types (see http://haxe.org/manual/types-abstract-array-access.html).
 **/
 extern interface ArrayAccess<T> { }