Add comment about Memory<T>

Author: egregius313

csharp/ql/lib/ext/System.IO.model.yml

@@ -92,10 +92,12 @@ extensions:
       - ["System.IO", "TextReader", True, "Read", "(System.Char[],System.Int32,System.Int32)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
       - ["System.IO", "TextReader", True, "Read", "(System.Span<System.Char>)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
       - ["System.IO", "TextReader", True, "ReadAsync", "(System.Char[],System.Int32,System.Int32)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
+      # Post-update nodes for `Memory<T>` are currently unsupported. This model is provided for completeness
       - ["System.IO", "TextReader", True, "ReadAsync", "(System.Memory<System.Char>,System.Threading.CancellationToken)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
       - ["System.IO", "TextReader", True, "ReadBlock", "(System.Char[],System.Int32,System.Int32)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
       - ["System.IO", "TextReader", True, "ReadBlock", "(System.Span<System.Char>)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
       - ["System.IO", "TextReader", True, "ReadBlockAsync", "(System.Char[],System.Int32,System.Int32)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
+      # Post-update nodes for `Memory<T>` are currently unsupported. This model is provided for completeness
       - ["System.IO", "TextReader", True, "ReadBlockAsync", "(System.Memory<System.Char>,System.Threading.CancellationToken)", "", "Argument[this]", "Argument[0].Element", "taint", "manual"]
       - ["System.IO", "TextReader", True, "ReadLine", "()", "", "Argument[this]", "ReturnValue", "taint", "manual"]
       - ["System.IO", "TextReader", True, "ReadLineAsync", "()", "", "Argument[this]", "ReturnValue", "taint", "manual"]