Add missing documentation for new 9.0 APIs in System.IO (dotnet#107376)

Author: jozkee

src/libraries/System.Private.CoreLib/src/System/IO/File.cs

@@ -819,6 +819,7 @@ public static void AppendAllBytes(string path, ReadOnlySpan<byte> bytes)
         /// <param name="path">The file to append to.</param>
         /// <param name="bytes">The bytes to append to the file.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation requests. The default value is <see cref="CancellationToken.None"/>.</param>
+        /// <returns>A task that represents the asynchronous append operation.</returns>
         /// <exception cref="ArgumentNullException"><paramref name="path"/> is null.</exception>
         /// <exception cref="ArgumentNullException"><paramref name="bytes"/> is null.</exception>
         /// <exception cref="ArgumentException"><paramref name="path"/> is empty.</exception>
@@ -837,6 +838,7 @@ public static Task AppendAllBytesAsync(string path, byte[] bytes, CancellationTo
         /// <param name="path">The file to append to.</param>
         /// <param name="bytes">The bytes to append to the file.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation requests. The default value is <see cref="CancellationToken.None"/>.</param>
+        /// <returns>A task that represents the asynchronous append operation.</returns>
         /// <exception cref="ArgumentNullException"><paramref name="path"/> is null.</exception>
         /// <exception cref="ArgumentException"><paramref name="path"/> is empty.</exception>
         /// <exception cref="OperationCanceledException">The cancellation token was canceled. This exception is stored into the returned task.</exception>

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs

@@ -536,6 +536,7 @@ public override void CopyTo(Stream destination, int bufferSize)
             _strategy.CopyTo(destination, bufferSize);
         }
 
+        /// <inheritdoc />
         public override Task CopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
         {
             ValidateCopyToArguments(destination, bufferSize);