(SERVER-253) Documentation for subcommands

Considering that most of these commands are standard Ruby ones I didn't feel the need to go into much detail. This ended up being mostly about showing examples of each one and calling out the `--` when running from source.

Happy to go change any/all of this; I just wanted to get something up before the break!
@nfagerlund This is an educated guesstimate of what should be here - please correct anything!