C++: Add provenance to MaD format explanation

Author: owen-mc

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

@@ -99,6 +99,15 @@
  *    sources "remote" indicates a default remote flow source, and for summaries
  *    "taint" indicates a default additional taint step and "value" indicates a
  *    globally applicable value-preserving step.
+ * 10. The `provenance` column is a tag to indicate the origin and verification of a model.
+ *    The format is {origin}-{verification} or just "manual" where the origin describes
+ *    the origin of the model and verification describes how the model has been verified.
+ *    Some examples are:
+ *    - "df-generated": The model has been generated by the model generator tool.
+ *    - "df-manual": The model has been generated by the model generator and verified by a human.
+ *    - "manual": The model has been written by hand.
+ *    This information is used in a heuristic for dataflow analysis to determine, if a
+ *    model or source code should be used for determining flow.
  */
 
 import cpp