GetSimpleDisplayText: explain how it works (#40065)

Add a `remarks` section with an explanation.  The explanation is high-level and does not convey all the gory details.

src/Mvc/Mvc.ViewFeatures/src/ModelExplorerExtensions.cs

@@ -17,6 +17,15 @@ public static class ModelExplorerExtensions
     /// </summary>
     /// <param name="modelExplorer">The <see cref="ModelExplorer"/>.</param>
     /// <returns>A simple display string for the model.</returns>
+    /// <remarks>
+    /// The result is obtained from the following sources (the first success wins):
+    /// <see cref="P:Microsoft.AspNetCore.Mvc.ModelBinding.ModelMetadata.SimpleDisplayProperty" />,
+    /// <see cref="ModelExplorer.Model" /> converted to string (if the result is interesting),
+    /// the first internal property converted to string,
+    /// <see cref="P:Microsoft.AspNetCore.Mvc.ModelBinding.ModelMetadata.NullDisplayText"  />
+    /// (when the value is <see langword="null" />).
+    /// This method is not recursive in order to prevent an infinite loop.
+    /// </remarks>
     public static string GetSimpleDisplayText(this ModelExplorer modelExplorer)
     {
         if (modelExplorer == null)