Improve documentation about the semantic of Immediate Services

[activeeon-bot]

Original issue created by Laurent Pellegrino on 18, Aug 2014 at 15:12 PM - PALIGHT-67

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It is not clearly explained inside the Javadoc (PAActiveObject.setImmediateService and org.objectweb.proactive.Body) and the ProActive documentation that using immediate services break the Proactive semantic on the caller side (i.e. even if the method has a return type declared as void or if the method returns a refiable object and does not throw an exception, any call is synchronous. In other words, all the calls performed on an active object with a method declared as IS are synchronous!).