Docs: OMP_NUM_THREADS also affects non-OpenMP builds

[takluyver]

I had a brief bit of confusion because the docs suggested that OMP_NUM_THREADS would only affect OpenMP builds, when actually it's used as a fallback in non-OpenMP builds as well.

https://github.com/OpenMathLib/OpenBLAS/blob/3a9da520d57c220a8fb3cf4166e1e77d73936566/driver/others/init.c#L825-L834