RFR 8131019: jshell tool: access javadoc from tool

[Jan Lahoda]

Hello,

This patch adds ability to show javadoc inside JShell (by showingg it on 
the second invocation of the Shift-<tab> documentation). In addition to 
jdk.jshell changes, there is a support in jdk.compiler, that is expected 
to be reused by jjs.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8131019

Webrev:
http://cr.openjdk.java.net/~jlahoda/8131019/webrev.00/

Any feedback is welcome!

Thanks,
     Jan