Add and correct comments

Also mandating this by generating a documentation XML file

Author: edwardneal

src/Microsoft.Data.SqlClient/tests/UnitTests/InternalsVisibleToTest.cs

@@ -4,8 +4,14 @@
 
 namespace Microsoft.Data.SqlClient.UnitTests
 {
+    /// <summary>
+    /// Tests proving that the InternalsVisibleTo attribute works correctly.
+    /// </summary>
     public class InternalsVisibleToTest
     {
+        /// <summary>
+        /// Creates an instance of an internal class. Verifies that this compiles.
+        /// </summary>
         [Fact]
         public void TestInternalsVisible()
         {

src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft.Data.SqlClient.UnitTests.csproj

@@ -7,6 +7,7 @@
     <IntermediateOutputPath>$(ObjFolder)$(Configuration).$(Platform).$(AssemblyName)</IntermediateOutputPath>
     <OutputPath>$(BinFolder)$(Configuration).$(Platform).$(AssemblyName)</OutputPath>
     <IsTestProject>true</IsTestProject>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
   </PropertyGroup>
   <!-- Common references -->
   <ItemGroup>

src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPoolTest.cs

@@ -11,6 +11,7 @@
 
 namespace Microsoft.Data.SqlClient.UnitTests
 {
+#pragma warning disable CS1591 // Test classes do not require XML documentation comments
     public class ChannelDbConnectionPoolTest
     {
         private readonly ChannelDbConnectionPool _pool;
@@ -152,4 +153,5 @@ public void TestTryGetConnection()
             Assert.Throws<NotImplementedException>(() => _pool.TryGetConnection(null!, null!, null!, out _));
         }
     }
+#pragma warning restore CS1591
 }

src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/UdtSerialization/InvalidSerializationTest.cs

@@ -11,8 +11,14 @@
 
 namespace Microsoft.Data.SqlClient.UnitTests.UdtSerialization;
 
+/// <summary>
+/// Attempts to serialize types which do not meet the requirements for either user-defined or native serialization.
+/// </summary>
 public class InvalidSerializationTest
 {
+    /// <summary>
+    /// Attempts to serialize a class that does not have the SqlUserDefinedType attribute. Verifies that this fails.
+    /// </summary>
     [Fact]
     public void RequiresSqlUserDefinedTypeAttribute()
     {
@@ -24,6 +30,10 @@ public void RequiresSqlUserDefinedTypeAttribute()
         Assert.Equal($"'{typeof(ClassMissingSqlUserDefinedTypeAttribute).FullName}' is an invalid user defined type, reason: no UDT attribute.", exception.Message);
     }
 
+    /// <summary>
+    /// Attempts to serialize a class that has a SqlUserDefinedType attribute, but specifies a Format enumeration value of
+    /// Unknown. Verifies that this fails.
+    /// </summary>
     [Fact]
     public void CannotSerializeUnknownFormattedType()
     {

src/Microsoft.Data.SqlClient/tests/UnitTests/Microsoft/Data/SqlClient/UdtSerialization/NativeSerializationTest.cs

@@ -18,6 +18,11 @@ namespace Microsoft.Data.SqlClient.UnitTests.UdtSerialization;
 /// <seealso href="https://learn.microsoft.com/en-us/openspecs/sql_server_protocols/ms-ssclrt/77460aa9-8c2f-4449-a65e-1d649ebd77fa"/>
 public class NativeSerializationTest
 {
+    /// <summary>
+    /// Provides a collection of test data representing non-null primitive type values and their corresponding
+    /// serialized byte arrays.
+    /// </summary>
+    /// <see cref="SerializePrimitiveType"/>
     public static IEnumerable<object[]> SerializedNonNullPrimitiveTypeValues()
     {
         yield return [new BoolWrapperStruct { Field1 = true },
@@ -62,6 +67,10 @@ public static IEnumerable<object[]> SerializedNonNullPrimitiveTypeValues()
             new byte[] { 0x01, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x2A, 0xF8 }];
     }
 
+    /// <summary>
+    /// Provides a collection of test data representing serialized values of nested non-null primitive types.
+    /// </summary>
+    /// <see cref="SerializeNestedPrimitiveType"/>
     public static IEnumerable<object[]> SerializedNestedNonNullPrimitiveTypeValues()
     {
         yield return [new NestedBoolWrapperStruct { Field1 = true, Field2 = new BoolWrapperStruct { Field1 = false } },
@@ -126,6 +135,10 @@ public static IEnumerable<object[]> SerializedNestedNonNullPrimitiveTypeValues()
                 0x01, 0x7F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x9C, 0x64 }];
     }
 
+    /// <summary>
+    /// Provides a collection of test data representing serialized null values for various primitive types.
+    /// </summary>
+    /// <see cref="SerializeNullPrimitiveType"/>
     public static IEnumerable<object[]> SerializedNullPrimitiveTypeValues()
     {
         yield return [new SqlByteWrapperStruct { Field1 = SqlByte.Null },
@@ -182,7 +195,7 @@ public void SerializeNullPrimitiveType(object primitive, byte[] expectedValue)
         => RoundtripType(primitive, expectedValue);
 
     /// <summary>
-    /// Attempts to serializes an instance of a class.
+    /// Attempts to serialize an instance of a class.
     /// </summary>
     /// <seealso cref="CannotSerializeNestedClass"/>
     [Fact]
@@ -199,11 +212,10 @@ public void CanSerializeTopLevelClass()
     }
 
     /// <summary>
-    /// Attempts to serializes a field referring to an instance of a class.
-    /// Verifies that this succeeds, and that Native format serialization only operates with primitive types
-    /// and value types containing these.
+    /// Attempts to serialize a field referring to an instance of a class.
+    /// Verifies that this fails, and that Native format serialization only operates with primitive types and value types containing these.
     /// </summary>
-    /// <seealso cref="CannotSerializeNestedClass"/>
+    /// <seealso cref="CanSerializeTopLevelClass"/>
     [Fact]
     public void CannotSerializeNestedClass()
     {