RFR JDK-8169561: jshell tool: double shift-tab on variable crashes tool

[Jan Lahoda]

Hello,

Documentation comments handling for JShell has a few problems:
-crashes on unknown HTML tags
-crashes when the documentation is not found (because the "not found" 
message is handled as an HTML)
-provides a wrong signature for variables declared in JShell itself.

This webrev fixes the problems:
http://cr.openjdk.java.net/~jlahoda/8169561/webrev.00/

Bug: https://bugs.openjdk.java.net/browse/JDK-8169561

Any comments are welcome,
     Jan