[install] Notes on OCaml versions and Coq configuration

I’m only seeing the bug you already know, With native_compute, flambda slows Coq (a bit on Linux, a lot on Mac) · Issue #11107 · coq/coq · GitHub. (I haven’t experienced On OS X, Coq with native-compiler is too slow · Issue #11178 · coq/coq · GitHub but that’s also relevant).