docs(mimic-iv): add CHARTEVENTS_TIMING for charttime vs outtime

Explain why chartevents can fall after icustays.outtime (#1961).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Chessing234

mimic-iv/docs/CHARTEVENTS_TIMING.md

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+# Chartevents timing vs ICU stay windows
+
+Researchers often find `icu.chartevents.charttime` values **after**
+`icu.icustays.outtime` (or occasionally before `intime`). Large absolute counts
+are expected in the raw PhysioNet files and usually do **not** mean a broken
+`stay_id` join.
+
+See [issue #1961](https://github.com/MIT-LCP/mimic-code/issues/1961).
+
+## Why charttime can fall outside `[intime, outtime]`
+
+1. **Late charting.** Nurses and devices often document observations after the
+   patient has left the ICU; the row still carries the ICU `stay_id`.
+2. **Device / interface clocks.** Bedside monitors and EHR interfaces may stamp
+   times that disagree slightly with ADT `intime`/`outtime`.
+3. **Derived stay boundaries.** `icustays` windows are constructed from
+   transfer / careunit logic; charted data is not clipped to those windows in
+   the ETL that loads the CSV files.
+
+The public tables do not expose a "manual vs automatic" provenance flag for
+each `charttime`.
+
+## Practical guidance
+
+- Treat `icustays.intime` / `outtime` as the **administrative** ICU window.
+- For "in-ICU only" cohorts, filter explicitly, e.g.
+  `charttime >= intime AND charttime <= outtime`, and report how many rows you
+  dropped.
+- Prefer a sensitivity analysis that also keeps a short post-`outtime` grace
+  period (e.g. 1–6 hours) when studying labs or vitals that are often charted
+  late.
+- Do **not** rewrite `outtime` from the latest `charttime` inside the PhysioNet
+  loaders — that belongs in a documented derived concept if you need it.
+
+## Related notes
+
+- ED module: `mimic-iv-ed/docs/TIMING_NOTES.md` (`edstays.intime` vs pyxis)
+- Lab / OMR unit caveats: `mimic-iv/buildmimic/TABLE_NOTES.md` when present