`datetime` documentation regarding ISO8601 reduced precision and extend representation

[nhhollander]

Documentation

The documentation for date.fromisoformat(), and datetime.fromisoformat() refer to these function as as being able to accept "any valid ISO 8601 format", however there are a couple notable exceptions to this support beyond what is already mentioned in the documentation. I know that Python's date parsing code intentionally does not implement the full ISO 8601 specification for simplicity reasons, but I feel like this could be better conveyed by the current documentation.

Below is a summary of ISO 8601 date stamps that are not supported by the datetime module.

• Reduced Precision Date Representations (ISO8601-1:2019§5.2.2.2)
• Extended Date Representations (ISO8601-1:2019§5.2.2.3)
• Ordinal Date Representations (ISO8601-1:2019§5.2.3): This is already mentioned in the documentation
• Local Time Representations with Decimal Fraction (ISO8601-1:2019§5.3.1.4): This is already mentioned in the documentation

Linked PRs

• gh-105031: Clarify datetime documentation for ISO8601 #105049
• [3.12] gh-105031: Clarify datetime documentation for ISO8601 (GH-105049) #114866
• [3.11] gh-105031: Clarify datetime documentation for ISO8601 (GH-105049) #114867

[serhiy-storchaka]

Thank you for your contribution @nhhollander.