Add note about collapsing multiple rows into one

docs/codeql/codeql-language-guides/customizing-library-models-for-csharp.rst

@@ -192,6 +192,19 @@ The remaining values are used to define the ``access path``, the ``kind``, and t
 - The ninth value ``taint`` is the kind of the flow. ``taint`` means that taint is propagated through the call.
 - The tenth value ``manual`` is the provenance of the summary, which is used to identify the origin of the summary.
 
+It would also be possible to merge the two rows into one by using a comma separated list in the seventh value. This would be useful if the method has many arguments and the flow is the same for all of them.
+
+.. code-block:: yaml
+
+   extensions:
+     - addsTo:
+         pack: codeql/csharp-all
+         extensible: summaryModel
+       data:
+         - ["System", "String", False, "Concat", "(System.Object,System.Object)", "", "Argument[0,1]", "ReturnValue", "taint", "manual"]
+
+This row defines flow from both the first and the second argument to the return value. The seventh value ``Argument[0,1]`` is shorthand for specifying an access path to both ``Argument[0]`` and ``Argument[1]``.
+
 Example: Add flow through the ``Trim`` method
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 This example shows how the C# query pack models flow through a method for a simple case.