gh-116938: Fix dict.update docstring and remove redundant period from dict web documentation

[Prometheus3375]

Changes made:

1. Removed premature period introduced in gh-116938: Clarify documentation of dict and dict.update regarding the positional argument they accept #125213.
2. Added a similar update made for MutableMapping.update to dict.update docstring. Also changed dict/iterable to mapping/iterable as E can be indeed any mapping object, not just dict.

• Issue: dict and dict.update treat the first argument as a mapping when it has attribute keys without attribute __getitem__ #116938

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📚 Documentation preview 📚: https://cpython-previews--125421.org.readthedocs.build/