docs(retail): add region tags for several samples

Added region tags for Search with pagination sample for CS.

Author: rebeccaellis

retail/interactive-tutorial/RetailSearch.Samples/SearchSimpleQuerySample.cs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// [START retail_search_simple_query]
+
 using Google.Cloud.Retail.V2;
 using System;
 using System.Collections.Generic;
@@ -106,3 +108,5 @@ public static IEnumerable<SearchResponse> Search()
         return sample.Search(projectId);
     }
 }
+
+// [END retail_search_simple_query]

retail/interactive-tutorial/RetailSearch.Samples/SearchWithPaginationSample.cs

@@ -16,6 +16,8 @@
 // limit the number of the products per page and go to the next page using "NextPageToken"
 // or jump to chosen page using "Offset".
 
+// [START retail_search_for_products_with_pagination]
+
 using Google.Cloud.Retail.V2;
 using System;
 using System.Collections.Generic;
@@ -105,6 +107,8 @@ public IEnumerable<SearchResponse> Search(string projectId)
     }
 }
 
+// [END retail_search_for_products_with_pagination]
+
 /// <summary>
 /// Search with page size tutorial.
 /// </summary>