OPENDJ-1242 (CR-5940) Enable dsconfig to generate doc for properties changed through subcommand options

Left code generating the reference documentation in opendj-cli SubCommandArgumentParser, so it can be reused with other tools using SubCommandArgumentParser.
Created SubCommandUsageHandler interface to allow outputing additional documentation + implemented it in opendj-config DSConfig, this way the code can access all the config classes and generate the reference documentation inline with the command usage.


SubCommandArgumentParser.java:
Added subCommandUsageHandler field + setter.
In toRefSect2(), used subCommandUsageHandler to output additional documentation.

DSConfig.java:
Created inner class DSConfigSubCommandUsageHandler + copied code from ConfigGuideGeneration to here.