[docs] [issue31937] Add the term "dunder" to the glossary

[Stéphane Wirtel]

Change by Stéphane Wirtel <stephane at wirtel.be>:


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keywords: +patch
pull_requests: +5399
stage: needs patch -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31937>
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