C#: Update change note with info on private fields.

Author: michaelnebel

csharp/ql/lib/change-notes/2024-12-03-public-protected-reference.md

@@ -1,4 +1,4 @@
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 category: minorAnalysis
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-* Only extract *public* and *protected* members from reference assemblies. This yields an approximate average speed-up of around 10% for extraction and query execution.
+* Only extract *public* and *protected* members from reference assemblies. This yields an approximate average speed-up of around 10% for extraction and query execution. Custom MaD rows using `Field`-based summaries may need to be changed to `SyntheticField`-based flows if they reference private fields.