RFR 8215099: jshell tool: /help representation of ctrl/meta characters inconsistent

[Robert Field]

Please review:

Fix the /help in-tool documentation -- specifically:
Make the representations of keys (Shift, Ctrl, Tab, etc) self-consistent 
and consistent with the User's Guide.
Make descriptions less cryptic.
Update and correct information.
Make formatting consistent.

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8215099

Webrev:

    http://cr.openjdk.java.net/~rfield/8215099v0.webrev/

Thanks,
Robert