[docs] [issue29974] Change typing.TYPE_CHECKING doc example

[Mathias Rav]

Mathias Rav added the comment:

I have updated the patch after feedback from Jelle Zijlstra and Ivan Levkevskyi. Indeed the example was correct; the patch now adds wording from PEP 484 and PEP 526 to clarify why a function parameter annotation needs to be in quotes and a local variable annotation does not.

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<http://bugs.python.org/issue29974>
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