Inconsistent documentation for build parallelism

[NobodyXu]

The crate-level doc says that:

If NUM_JOBS is not set, the RAYON_NUM_THREADS environment variable can
also specify the build parallelism.

However in default_jobserver(), only NUM_JOBS is used but not RAYON_NUM_THREADS.

It seems like this is an outdated documentation, but I'm not entirely sure so I opened this issue.

[thomcc]

Yeah, a long time ago we used rayon. This documentation is outdated though.

[NobodyXu]

I will open a PR to remove this.