Fix missing docs

[yoshuawuyts]

This PR adds a number of additional comments, mostly auto-generated. Most notably though: this documents the serve subcommand, which is important for people running the commands from the command line. With this PR it now looks like this:

Usage: wassette <COMMAND>

Commands:
  serve  Begin handling requests over the specified protocol
  help   Print this message or the help of the given subcommand(s)

Options:
  -h, --help     Print help
  -V, --version  Print version

[Mossaka]

Needs a rebase

[asw101]

@Mossaka will do

[asw101]

@yoshuawuyts I removed two extra comments. One was identical.