`JULIA_NUM_THREADS` doc should mention about interactive threads number configuration

[AtsushiSakai]

This is a small doc improvement idea.
From v1.9, interactive thread number can be configurable with JULIA_NUM_THREADS env

export JULIA_NUM_THREADS=3,1

Ref: https://docs.julialang.org/en/v1/manual/multi-threading/#man-threadpools

However, the latest doc about JULIA_NUM_THREADS does not mention it:

I think it should do it.

[nickrobinson251]

related #48960