Add comment to IDE Diagnostic ID for 'Use expression body for local functions'

src/Features/Core/Portable/Diagnostics/Analyzers/IDEDiagnosticIds.cs

@@ -99,6 +99,7 @@ internal static class IDEDiagnosticIds
         public const string ValueAssignedIsUnusedDiagnosticId = "IDE0059";
         public const string UnusedParameterDiagnosticId = "IDE0060";
 
+        // Conceptually belongs with IDE0021-IDE0027 & IDE0053, but is here because it was added later
         public const string UseExpressionBodyForLocalFunctionsDiagnosticId = "IDE0061";
 
         // Analyzer error Ids