gh-157747: Document that ** unpacking accepts any object with keys() - #157771

The reference says ** unpacking, both in dict displays and in calls, needs a mapping, and the glossary defines that as something implementing the full collections.abc.Mapping ABC. At runtime CPython only calls keys() and then __getitem__(), so objects like a pandas DataFrame work fine even though they are not mappings. This rewords both places to match what the interpreter does, using the same wording as the dict constructor and dict.update() docs.

Fixes #157747

• Issue: [docs] dictionary unpacking fn(**kwargs) should accept SupportsKeysAndGetitem and not just Mapping #157747