Add `stg branch --cleanup` as `see-also` section to `stg init` man page

[przemo-hemmersbach]

As in title. stg branch could be added to man page for stg init.
If PR would be accepted I can create one, but I failed to locate docs for stg-init in source code. Where is it?

[jpgrayson]

The man pages are generated from the strings provided to clap::App.long_about() / .about() and clap::Arg::long_help() / .help() calls.

So to improve the stg-init(1) man page, a .long_about() call in src/cmd/init.rs needs to be added.

I am noticing that I failed to port any of the long about from the Python implementation of stg init into the Rust implementation. I will correct that issue and add a statement about stg branch --cleanup while I'm at it. Please submit a follow-on PR if there are any further changes you'd like to see in the stg init documentation.

Thanks for pointing out this issue, @przemo-hemmersbach!