[docs] [issue23006] Improve the doc and indexing of adict.__missing__.

[Terry J. Reedy]

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


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keywords: +patch
Added file: http://bugs.python.org/file37383/__missing__.diff

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23006>
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