Support for test cases in documentation comments

[jpmckinney]

I use clap, and I put examples in the documentation comments for my commands/subcommands like:

enum Commands {
    /// Counts from START to STOP.
    ///
    /// $ mycli counter 1 3
    /// 1
    /// 2
    /// 3
    Counter {
        start: usize,
        stop: usize,
    }
}

From discussion in clap-rs/clap#4500

One alternative is to pull a markdown file into your doc comment

e.g. #[doc = include_str!("../../README.md")]