[docs] [issue23725] update tempfile docs to say that TemporaryFile is secure

[Roundup Robot]

Roundup Robot added the comment:

New changeset 51d00482d403 by Robert Collins in branch '3.5':
Issue #23725: Overhaul tempfile docs.
https://hg.python.org/cpython/rev/51d00482d403

New changeset 256d2f01e975 by Robert Collins in branch 'default':
Issue #23725: Overhaul tempfile docs.
https://hg.python.org/cpython/rev/256d2f01e975

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nosy: +python-dev

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