[3.14] gh-142518: Document thread-safety guarantees of dict operations (GH-144184) (#144708)

* Address feedback; move thread safety section below see-also
* Address feedback - don't mention equality comparison only
* Change admonition to rubric; cross-reference glossary

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(cherry picked from commit 35dc547ab5a6bb9be9748002d42d0d9e86f9cced)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>