docs(mimic-iv): explain where Vitamin C appears in MIMIC

Answer #1962: no dedicated table — use prescriptions/emar/labs.

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

Author: Chessing234

mimic-iv/docs/VITAMIN_C.md

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+# Finding Vitamin C (ascorbic acid) in MIMIC
+
+There is **no dedicated Vitamin C table** in MIMIC-III or MIMIC-IV.
+
+See [issue #1962](https://github.com/MIT-LCP/mimic-code/issues/1962).
+
+## Where exposure usually lives
+
+| Source | Module | What to search |
+|--------|--------|----------------|
+| `prescriptions` | hosp | drug / drug_name_generic containing `ascorbic`, `vitamin c`, `vit c` |
+| `emar` / `emar_detail` | hosp | administered meds with the same name patterns |
+| `inputevents` (III: `_cv` / `_mv`) | icu | infusions if charted as inputs (rare for vit C) |
+| `labevents` + `d_labitems` | hosp | serum ascorbic acid **levels** (not doses), if ordered |
+
+Retrospective papers that report “Vitamin C exposure” almost always derive a
+cohort from **medication orders / eMAR**, not from a first-class vit-C module.
+
+## Practical guidance
+
+1. Start with `prescriptions` / `emar` string filters; review false positives
+   (multivitamins, “C” abbreviations).
+2. Decide whether you need **orders**, **administrations**, or **serum labs** —
+   they answer different questions.
+3. Do not expect complete capture: OTCs and outside-hospital vitamins are often
+   absent.
+
+## What this repository will not add
+
+A curated “official” Vitamin C concept would need ongoing pharmacy vocabulary
+maintenance. Contribute a versioned derived concept with explicit inclusion
+strings if you build one for a paper.