[`flake8-datetimez`] Clarify naming guidance for `datetime.today` (`DTZ002`) (#26658)
## Summary
This updates the DTZ002 documentation to clarify the `datetime.today()`
naming confusion raised in #23392.
- Notes that `datetime.datetime.today()` is named like a date helper,
but it returns a local `datetime`.
- Keeps the existing guidance to use timezone-aware `datetime.now(...)`
when you need an aware timestamp.
## Why this changed
The code behavior is already correct; this is a docs-only improvement to
make the rule intent easier to understand.
## Test Plan
- `cargo dev generate-docs`
- `cargo fmt --check --all`
- `PATH="$PWD/target/debug:$PATH" uv run --only-group dev --locked
python scripts/check_docs_formatted.py`
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Co-authored-by: Mahadev Annabhimoju <219508079+Joosboy@users.noreply.github.com>
Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>