Any documentation about working on BuildStream itself should be covered in the HACKING file, which is expressly meant for details about hacking on BuildStream.
In the comment you refer to, I also mentioned that how to build the docs is covered in the HACKING file, this was more me being extra helpful to a new contributor by handing out the information directly in a comment.
Seeing as this is covered in the HACKING file already, I will close this.