Document importance of PyEval_EvalInitThreads()

[nikratio]

It would be great if  http://docs.cython.org/src/userguide/external_C_code.html#acquiring-and-releasing-the-gil could be extended with something like

" WARNING: If the C components of the application are starting threads before any Python code does so, the PyEval?_EvalInitThreads() needs to be called (to initialize Python thread support) before any "with gil" functions are invoked. "

in the "Acquiring the GIL" section.

See also  http://article.gmane.org/gmane.comp.python.cython.user/4207

[mark]

Fixed it a while ago in 8a7d02b61ac0d2cd72959060d7ef19f8955eda41