C#/Java: Add descriptive comment on negative summaries in ExternalFlow.

csharp/ql/lib/semmle/code/csharp/dataflow/ExternalFlow.qll

@@ -12,6 +12,8 @@
  *   `namespace; type; subtypes; name; signature; ext; input; output; kind; provenance`
  * - Negative Summaries:
  *   `namespace; type; name; signature; provenance`
+ *   A negative summary is used to indicate that there is no flow via a callable.
+ *
  * The interpretation of a row is similar to API-graphs with a left-to-right
  * reading.
  * 1. The `namespace` column selects a namespace.