RFR 8146253: jjs should look for "doc string" property to print documentation on shift-tab

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8146253/ for 
https://bugs.openjdk.java.net/browse/JDK-8146253

Note that this bug just adds printing __doc__ property where it is 
available (on shift-tab). There is no change to associate doc string to 
a function or an object. But, users can just define that property. If a 
user defined function is defined as follows, jjs prints __doc__ string 
when shift-tab is pressed after "func".

function func() { print("hello") }
func.__doc__ = "prints 'hello' in the standard output";

Thanks,
-Sundar