RFR 8145486: jjs should support documentation key shortcut in interactive mode

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8145486/ for 
https://bugs.openjdk.java.net/browse/JDK-8145486

Note that this enhancement only adds jjs changes to show documentation 
when available and infrastructure to attach documentation string for JS 
built-in functions, constructors.
Actual documentation strings are added only for NativeObject. Adding 
documentation string for all other built-in functions, constructors will 
be done in future.

Thanks,
-Sundar