Making possible several help-screens for different commands is on it's way. The only stopper—deciding on API: https://github.com/docopt/docopt/issues/17
But I'd strongly vote for making it possible to annotate multiple functions and merging their respective doc-strings smartly.
I don't think you'd want to express the entire syntax-tree of a reasonably complex program (e.g. 'git') in a single doc-string. Not only would that turn into a mess, but you'd also lose modularity. I.e. subcommands often need to be dynamically generated (e.g. depending on loaded modules) and not all subcommands may be available at all times.