Some RTS options to do with concurrency missing from doc page

For instance -maxN and -qa are documented on the "Using Concurrent Haskell" (https://downloads.haskell.org/ghc/latest/docs/users_guide/using-concurrent.html) page but are missing from the general RTS documentation options page (https://downloads.haskell.org/ghc/latest/docs/users_guide/runtime_control.html)

I think this is just a case of copying over the relevant documentation. I'm happy to do this.