It would be a nice convention if CLI applications shipped a USAGE.txt or OPTIONS.txt file written in docopt in the root of their repo, along other standard files that one expects to be there like README.txt, requirements.txt, setup.py, project.json, etc.
That way it would be easier to write utilities for testing the CLI, to generate the shell completion script and to change the interface without messing with hard-coded strings in the code.
There is a convention of providing concise documentation of options in `<command> --help` and more verbose documentation of options in the man page. When you look through a variety of common tools' documentation, you will notice that there are conventions that could almost be called a standard. But when you look carefully, you will find minor but important differences in how various tools write the options documentation -- enough to inhibit parsing it automatically.
That way it would be easier to write utilities for testing the CLI, to generate the shell completion script and to change the interface without messing with hard-coded strings in the code.