(copy editors) RFR 8179858: jshell tool: sync nomenclature from reference to online /help

[Robert Field]

This proceeded from a bug to sync /help with the nomenclature in the 
jshell Tool Reference.
In the process, fresh eyes also addressed other glaring issues in /help.
Please review, comments welcome from all.

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8179858

Webrev:

     http://cr.openjdk.java.net/~rfield/8179858v0.webrev/

Thanks,
Robert