docs(mimic-iv): sample d_items query for SpO2-like labels

Help #1972-style terminology work without inventing LOINC codes.

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

Author: Chessing234

mimic-iv/concepts/documentation/chartevents_item_sample.sql

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+-- Sample ICU d_items labels (vitals/scores) for manual terminology review (#1972).
+
+SELECT itemid, label, abbreviation, linksto, category
+FROM icu.d_items
+WHERE LOWER(label) LIKE '%spo2%'
+   OR LOWER(label) LIKE '%o2 saturation%'
+ORDER BY itemid
+LIMIT 50;