Address Shannon comments

Add code documentation, return empty enumerable instead of allocate empty list

src/Umbraco.Web/Search/IUmbracoTreeSearcherFields.cs

@@ -2,12 +2,26 @@ using System.Collections.Generic;
 
 namespace Umbraco.Web.Search
 {
+    /// <summary>
+    /// Used to propagate hardcoded internal Field lists
+    /// </summary>
     public interface IUmbracoTreeSearcherFields
     {
+        /// <summary>
+        /// Propagate list of searchable fields for all node types
+        /// </summary>
         IEnumerable<string> GetBackOfficeFields();
+        /// <summary>
+        /// Propagate list of searchable fields for Members
+        /// </summary>
         IEnumerable<string> GetBackOfficeMembersFields();
-
+        /// <summary>
+        /// Propagate list of searchable fields for Media
+        /// </summary>
         IEnumerable<string> GetBackOfficeMediaFields();
+        /// <summary>
+        /// Propagate list of searchable fields for Documents
+        /// </summary>
         IEnumerable<string> GetBackOfficeDocumentFields();
     }
 }

src/Umbraco.Web/Search/UmbracoTreeSearcherFields.cs

@@ -1,8 +1,8 @@
 using System.Collections.Generic;
+using System.Linq;
 using Umbraco.Examine;
-using Umbraco.Web.Search;
 
-namespace Umbraco.Web
+namespace Umbraco.Web.Search
 {
     public class UmbracoTreeSearcherFields : IUmbracoTreeSearcherFields
     {
@@ -23,10 +23,9 @@ namespace Umbraco.Web
         {
             return _backOfficeMediaFields;
         }
-        private IReadOnlyList<string> _backOfficeDocumentFields = new List<string> ();
         public IEnumerable<string> GetBackOfficeDocumentFields()
         {
-            return _backOfficeDocumentFields;
+            return Enumerable.Empty<string>();
         }
     }
 }