Documentation on Multiple GC Threads

[Sbozzolo]

As far as I can tell, using multiple GC threads is only documented in a short paragraph that brings up the existence of JULIA_NUM_GC_THREADS and --gcthreads.

The paragraph, and the documentation in general, does not explain how one would go about setting an optimal value for this variable. It would be great if the documentation answered questions like: Is larger always better? What factors should be taken into account when setting --gcthreads? Why is half the number of compute workers the default?