RFR 8177078 et. al. : three /help fixes

[Robert Field]

Please review:

Bugs:

     8177078: jshell tool: fix documentation of /help shortcuts
     https://bugs.openjdk.java.net/browse/JDK-8177078

     8177735: jshell tool: /help /help -- typo "comand"
     https://bugs.openjdk.java.net/browse/JDK-8177735

     8177308: jshell tool: documentation: multiple start-up files and 
predefines not documented
     https://bugs.openjdk.java.net/browse/JDK-8177308

Webrev:

     http://cr.openjdk.java.net/~rfield/8177078v0.webrev/

Thanks,
Robert