Rust: Add neutrals to MaD format explanation

Author: owen-mc

rust/ql/lib/codeql/rust/dataflow/internal/ModelsAsData.qll

@@ -13,6 +13,9 @@
  *   `path; input; kind; provenance`
  * - BarrierGuards:
  *   `path; input; branch; kind; provenance`
+ * - Neutrals:
+ *   `package; type; name; signature; kind; provenance`
+ *   A neutral is used to indicate that a callable is neutral with respect to flow (no summary), source (is not a source) or sink (is not a sink).
  *
  * The interpretation of a row is similar to API-graphs with a left-to-right
  * reading.