Improve documentation for def/custom command definition

[generic-github-user]

Some of the examples from the page on custom commands may be worth adapting to help provide more comprehensive usage information in the documentation on the def statement; particularly those covering subcommand definitions, parameter types, default parameter values, etc. The command description itself and parameter information could probably also be made more thorough. I'll put together a PR to this effect and you can decide whether it would be beneficial to integrate into the documentation.