[docs] [issue30535] Warn that meta_path is not empty

[Xavier Morel]

Xavier Morel added the comment:

> A warning is probably too strong.  Also, it's easy to check sys.meta_path at the interpreter prompt, so I'm not sure it's worth mentioning at all.

It's easy if you think of it and did not miss a small bit of the Python 3.3 release note indicating that the *documented guarantees* of Python 2.x, 3.0, 3.1 and 3.2 had been dropped, even as they were incorrectly still present in the official documentation itself.

I spent a few hours trying to understand why our import hooks did not work correctly anymore in 3.5, and I never even considered printing sys.meta_path in an open interpreter, I realised the behavioural change because I ended up printing meta_path after the addition of our custom hooks to make sure they were actually there.

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<http://bugs.python.org/issue30535>
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