Added some docs and a bugfix on debugging a JVM tool (like jar-tool or checkstyle) locally

Testing Done:
To test the new debugging tool, I edited pants.ini as follows:

[jar-tool]
    jvm_args: ['-Xmx300m', '-Xdebug', '-Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=%(debug_port)s']

Then attached to it with IntelliJ using a remote debug config for the twitter-commons repo

Bugs closed: 413

Reviewed at https://rbcommons.com/s/twitter/r/791/