[issue21603] IDLE: Document SaveAs extension display on Mac

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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assignee:  -> terry.reedy
stage:  -> test needed
versions: +Python 3.6, Python 3.7 -Python 2.7, Python 3.4, Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21603>
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