docs(mimic-iv): sample prescriptions search for Vitamin C

Give #1962 researchers a starting drug-name query.

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

Author: Chessing234

mimic-iv/concepts/documentation/vitamin_c_prescriptions.sql

@@ -0,0 +1,11 @@
+-- Candidate Vitamin C / ascorbic acid orders in hosp.prescriptions (#1962).
+-- Review hits manually — multivitamins and abbreviations create false positives.
+
+SELECT drug, COUNT(*) AS n
+FROM hosp.prescriptions
+WHERE LOWER(drug) LIKE '%ascorbic%'
+   OR LOWER(drug) LIKE '%vitamin c%'
+   OR LOWER(drug) LIKE '%vit c%'
+GROUP BY drug
+ORDER BY n DESC
+LIMIT 50;