docs(mimic-iv): sample d_labitems query for manual LOINC review

Show researchers where labels live when no official map exists (#1972).

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

Author: Chessing234

mimic-iv/concepts/documentation/labitems_label_sample.sql

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+-- Sample d_labitems labels for cohorts that need a manual LOINC review (#1972).
+-- There is no LOINC column in MIMIC-IV dictionaries.
+
+SELECT itemid, label, fluid, category
+FROM hosp.d_labitems
+WHERE LOWER(label) LIKE '%creatinine%'
+ORDER BY itemid
+LIMIT 50;