DOC: site.cfg: warn against OpenBLAS and multiprocessing

Fixes #654 by not fixing it; I don't think NumPy *can* actually fix
the problem as it's a design flaw in Python's multiprocessing. Listed
various alternatives (Python 3.4 forkserver, single-threaded OpenBLAS,
Python threading).
Merge pull request #4194 from larsmans/openblas-docs

DOC: site.cfg: warn against OpenBLAS and multiprocessing