I’m not sure why people are downvoting you. I suspect they may be clicking the link and thinking ‘that’s not documentation it’s source code’, not realizing it actually _is_ documentation.

The language it’s in is ‘mdoc’ - a markup format for man pages: https://man.freebsd.org/cgi/man.cgi?mdoc

It’s the source code for the man page, which is about as documentationey as you can get.