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.

OpenBLAS/driver/others/init.c

Lines 825 to 834 in 3a9da52

	#ifdef USE_OPENMP
	numprocs = 0;
	#else
	numprocs = readenv_atoi("OPENBLAS_NUM_THREADS");
	if (numprocs == 0) numprocs = readenv_atoi("GOTO_NUM_THREADS");
	#endif
	if (numprocs == 0) numprocs = readenv_atoi("OMP_NUM_THREADS");
	if (numprocs == 0) numprocs = readenv_atoi("OPENBLAS_DEFAULT_NUM_THREADS");