docs: document date_bin DST drift with an explicit origin - #27381

Which issue does this PR close?

• Closes Document that date_bin with an explicit origin drifts an hour across a DST transition #25168.

Rationale for this change

An explicit origin_timestamp chosen to align date_bin bins to local midnight works until the timezone's offset changes, then silently shifts the bins by an hour. There is no error and no warning. This is not a bug in DataFusion — PostgreSQL and DuckDB return identical results, because all three bin on the instant measured from a fixed origin — but the pattern looks correct in every test that does not cross a DST transition, so the issue asks for it to be documented.

What changes are included in this PR?

Adds a short note to the date_bin documentation: an explicit origin fixes an instant, not a recurring local time, so bins aligned to local midnight with an origin silently shift by an hour across a daylight saving time transition. It points users at date_trunc for calendar units, and at date_bin(<stride>, to_local_time(t AT TIME ZONE '<zone>')) for strides that are not calendar units.

The note is added to the rustdoc in datafusion/functions/src/datetime/date_bin.rs and mirrored into docs/source/user-guide/sql/scalar_functions.md (which dev/update_function_docs.sh regenerates from the rustdoc).

What is the testing strategy for this PR?

Docs-only change; no code or test behaviour is touched.

• cargo test -p datafusion-functions --lib — 410 passed, 0 failed.
• cargo fmt --check on the touched crate.

Are there any user-facing changes?

Documentation only. No behaviour change, no public API change.