Update SA1004 to allow doc comments to start with the readonly keyword

#3817

Author: bjornhellander

StyleCop.Analyzers/StyleCop.Analyzers.Test.CSharp12/SpacingRules/SA1004CSharp12UnitTests.cs

@@ -3,9 +3,35 @@
 
 namespace StyleCop.Analyzers.Test.CSharp12.SpacingRules
 {
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis.Testing;
     using StyleCop.Analyzers.Test.CSharp11.SpacingRules;
+    using Xunit;
+    using static StyleCop.Analyzers.Test.Verifiers.StyleCopCodeFixVerifier<
+        StyleCop.Analyzers.SpacingRules.SA1004DocumentationLinesMustBeginWithSingleSpace,
+        StyleCop.Analyzers.SpacingRules.SA1004CodeFixProvider>;
 
     public partial class SA1004CSharp12UnitTests : SA1004CSharp11UnitTests
     {
+        [Fact]
+        [WorkItem(3817, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3817")]
+        public async Task TestParameterModifierReadOnlyFirstOnLineAsync()
+        {
+            string testCode = $@"
+/// <summary>
+/// Description of some remarks that refer to a method: <see cref=""SomeMethod(int, int, ref
+/// readonly string)""/>.
+/// </summary>
+public class TypeName
+{{
+    public void SomeMethod(int x, int y, ref readonly string z)
+    {{
+        throw new System.Exception();
+    }}
+}}";
+
+            await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+        }
     }
 }

StyleCop.Analyzers/StyleCop.Analyzers/SpacingRules/SA1004DocumentationLinesMustBeginWithSingleSpace.cs

@@ -113,6 +113,7 @@ private static void HandleDocumentationCommentExteriorTrivia(SyntaxTreeAnalysisC
             case SyntaxKind.InKeyword:
             case SyntaxKind.OutKeyword:
             case SyntaxKind.RefKeyword:
+            case SyntaxKind.ReadOnlyKeyword:
                 if (!token.HasLeadingTrivia)
                 {
                     break;