This is the home of the Language Server project docs. The product is very basic so far, but more investment will come (said in early 2022).
In the meantime, there is a BazelCon presentation you can watch that covers the ideas. Start at the 12 minute mark to see the Language Server portion of the talk:
Overall, as of November 2022, the BJLS is still a prototype. There isn't a roadmap or docuementation and there isn't active development.