More useful documentation on /help

From:
http://mail.openjdk.java.net/pipermail/kulla-dev/2017-March/001978.html

Second, when completing inside help subcommands, the "todo" list is packed with generic
"help on help", which promises information but isn't that useful. If you are already saying
"/help intro <Tab>" it doesn't seem very helpful to back up to the same help text as
"/help <Tab><Tab>".

A further example of the odd help-doc is below.

— John

jshell> /help /set tr<Tab>
===REDRAW===>
jshell> /help /set truncation<Tab>
<press tab again to see synopsis>

jshell> /help /set truncation<Tab>
get information about jshell <==== NOT A SYNOPSIS OF "truncation"

<press tab again to see full documentation>

jshell> /help /set truncation<Tab>
Display information about jshell. <==== NOT FULL DOC OF "truncation"
/help
    List the jshell commands and help subjects.

/help <command>
    Display information about the specified command. The slash must be included.
    Only the first few letters of the command are needed -- if more than one
    each will be displayed. Example: /help /li

/help <subject>
    Display information about the specified help subject. Example: /help intro

jshell>