Add neutral model note to C# documentation

Author: egregius313

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

@@ -58,7 +58,7 @@ The CodeQL library for C# analysis exposes the following extensible predicates:
 - ``sourceModel(namespace, type, subtypes, name, signature, ext, output, kind, provenance)``. This is used to model sources of potentially tainted data.
 - ``sinkModel(namespace, type, subtypes, name, signature, ext, input, kind, provenance)``. This is used to model sinks where tainted data may be used in a way that makes the code vulnerable.
 - ``summaryModel(namespace, type, subtypes, name, signature, ext, input, output, kind, provenance)``. This is used to model flow through elements.
-- ``neutralModel(namespace, type, name, signature, kind, provenance)``. This is similar to a summary model but used to model the flow of values that have only a minor impact on the dataflow analysis.
+- ``neutralModel(namespace, type, name, signature, kind, provenance)``. This is similar to a summary model but used to model the flow of values that have only a minor impact on the dataflow analysis. Manual neutral models (those with a provenance such as ``manual`` or ``ai-manual``) can be used to override generated summary models (those a provenance such as ``df-generated``), such that the summary model will be ignored. Other than that, neutral models have no effect.
 
 The extensible predicates are populated using the models defined in data extension files.