[docs] [issue12920] Document that inspect.getsource only works for objects loaded from files, not interactive session

[Ned Deily]

Ned Deily added the comment:

In duplicate Issue24491, zorceta notes: "Both python.exe and IDLE can't. IPython is able to, as it inserts REPL input into linecache."

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nosy: +ned.deily, zorceta
versions: +Python 3.5, Python 3.6 -Python 3.2, Python 3.3

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<http://bugs.python.org/issue12920>
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