Add missing <returns> to AppendAllBytesAsync (#10353)

Contributes to dotnet/runtime#105979 (comment)

Author: jozkee

xml/System.IO/File.xml

@@ -295,7 +295,7 @@
           <para>Asynchronously appends the specified byte array to the end of the file at the given path.</para>
           <para>If the file doesn't exist, this method creates a new file. If the operation is canceled, the task will return in a canceled state.</para>
         </summary>
-        <returns>To be added.</returns>
+        <returns>A task that represents the asynchronous append operation.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="path" /> or <paramref name="bytes" /> is <see langword="null" />.
@@ -346,7 +346,7 @@
           <para>If the file doesn't exist, this method creates a new file. If the operation is canceled, the task will return in a canceled state.</para>
         </summary>
         <returns>To be added.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>A task that represents the asynchronous append operation.</remarks>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="path" /> is <see langword="null" />.</exception>
         <exception cref="T:System.ArgumentException">