docs: document default values for Filters properties in XML docs

Continues the work started for #5917 (already covered Targets, Layouts,
LayoutRenderers and Target Wrappers), extending the same pattern to the
NLog.Filters namespace so default values show up in IntelliSense and the
generated API docs.

Adds a regression test that scans all settable NLog.Filters properties
against the generated XML documentation file and fails if any of them is
missing a documented default value.

tests/NLog.UnitTests/Filters/FilterDefaultValueDocsTests.cs

@@ -0,0 +1,95 @@
+//
+// Copyright (c) 2004-2024 Jaroslaw Kowalski <jaak@jkowalski.net>, Kim Christensen, Julian Verdurmen
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions
+// are met:
+//
+// * Redistributions of source code must retain the above copyright notice,
+//   this list of conditions and the following disclaimer.
+//
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution.
+//
+// * Neither the name of Jaroslaw Kowalski nor the names of its
+//   contributors may be used to endorse or promote products derived from this
+//   software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+// THE POSSIBILITY OF SUCH DAMAGE.
+//
+
+namespace NLog.UnitTests.Filters
+{
+    using System;
+    using System.Collections.Generic;
+    using System.IO;
+    using System.Linq;
+    using System.Reflection;
+    using System.Xml.Linq;
+    using Xunit;
+
+    /// <summary>
+    /// Verifies that every settable property under the <see cref="NLog.Filters"/> namespace
+    /// documents its default value in the generated XML docs, so it shows up in IntelliSense
+    /// and the Sandcastle API docs. See https://github.com/NLog/NLog/issues/5917
+    /// </summary>
+    public class FilterDefaultValueDocsTests : NLogTestBase
+    {
+        // Properties that intentionally do not document a default value.
+        private static readonly HashSet<string> ExcludedMembers = new HashSet<string>
+        {
+            // Obsolete since NLog 5.0, always returns true, no meaningful default to document.
+            "P:NLog.Filters.WhenRepeatedFilter.OptimizeBufferReuse",
+        };
+
+        [Fact]
+        public void SettablePropertiesDocumentDefaultValue()
+        {
+            var nlogAssembly = typeof(LogManager).Assembly;
+            var xmlDocPath = Path.ChangeExtension(nlogAssembly.Location, ".xml");
+            Assert.True(File.Exists(xmlDocPath), $"Generated XML doc file not found at '{xmlDocPath}'. Build NLog.csproj with GenerateDocumentationFile enabled.");
+
+            var documentedMembers = XDocument.Load(xmlDocPath)
+                .Descendants("member")
+                .ToDictionary(member => (string)member.Attribute("name"), member => member.ToString());
+
+            var propertiesMissingDefaultValueDocs = new List<string>();
+
+            foreach (var type in nlogAssembly.GetTypes())
+            {
+                if (type.Namespace != "NLog.Filters" || !type.IsPublic)
+                    continue;
+
+                foreach (var property in type.GetProperties(BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly))
+                {
+                    if (!property.CanWrite || property.GetCustomAttribute<ObsoleteAttribute>() != null)
+                        continue;
+
+                    var memberName = $"P:{type.FullName}.{property.Name}";
+                    if (ExcludedMembers.Contains(memberName))
+                        continue;
+
+                    if (!documentedMembers.TryGetValue(memberName, out var memberDocs) || !memberDocs.Contains("Default:"))
+                    {
+                        propertiesMissingDefaultValueDocs.Add(memberName);
+                    }
+                }
+            }
+
+            Assert.Empty(propertiesMissingDefaultValueDocs);
+        }
+    }
+}