improved docs for constraints on compound bodies, closes #442

src/constraint/Constraint.js

@@ -30,6 +30,7 @@ var Common = require('../core/Common');
      * All properties have default values, and many are pre-calculated automatically based on other properties.
      * To simulate a revolute constraint (or pin joint) set `length: 0` and a high `stiffness` value (e.g. `0.7` or above).
      * If the constraint is unstable, try lowering the `stiffness` value and / or increasing `engine.constraintIterations`.
+     * For compound bodies, constraints must be applied to the parent body (not one of its parts).
      * See the properties section below for detailed information on what you can pass via the `options` object.
      * @method create
      * @param {} options