Update doc comment for RazorInjectAttribute (#11554)

* Update doc comment for RazorInjectAttribute

src/Mvc/Mvc.Razor/src/Internal/RazorInjectAttribute.cs

@@ -5,8 +5,15 @@ using System;
 
 namespace Microsoft.AspNetCore.Mvc.Razor.Internal
 {
+    /// <summary>
+    /// Specifies that the attributed property should be bound using request services.
+    /// <para>
+    /// This attribute is used as the backing attribute for the <code>@inject</code>
+    /// Razor directive.
+    /// </para>
+    /// </summary>
     [AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
     public class RazorInjectAttribute : Attribute
     {
     }
-}
+}