[issue39267] Fix dict's __missing__ documentation

Change by Bar Harel : -- keywords: +patch pull_requests: +17322 stage: -> patch review pull_request: https://github.com/python/cpython/pull/17911 ___ Python tracker ___

[issue39267] Fix dict's __missing__ documentation

New submission from Bar Harel : Continuing bpo-39264, and according to the mailing list discussion at Python-Dev. Fixing dict's __missing__ documentation. Clarify .get() does not call __missing__, and move __missing__ from the data model to dict's section as it's not a general object or ABC