[docs] [issue17546] Document the circumstances where the locals() dict gets updated

[anatoly techtonik]

anatoly techtonik added the comment:

Example:

    l = locals()
    z = dict(a=5, b=3)

    lc = dict(l)
    zc = dict(z)

    print(lc == l)
    print(zc == z)

Gives:

    False
    True

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17546>
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