Add documentation for indexing batch size configuration

Author: kjac

17/umbraco-cms/reference/configuration/indexingsettings.md

@@ -10,7 +10,8 @@ This section allows you to configure how content is indexed for Examine.
 "Umbraco": {
   "CMS": {
     "Indexing": {
-      "ExplicitlyIndexEachNestedProperty": true
+      "ExplicitlyIndexEachNestedProperty": true,
+      "BatchSize": 10000
     }
   }
 }
@@ -25,3 +26,9 @@ When indexing content, each property contained within certain complex editors ar
 The complex editors are also indexed to their own separate fields, which then contains "the sum" of all properties contained within.
 
 In some cases this yields a lot of fields in the index, which can lead to errors when performing searches. Changing this setting to `false` can mend that issue. It prevents each contained property from being written to the index in its own field.
+
+## BatchSize
+
+For large indexing operations (e.g. rebuilding an index), content is loaded from the database and indexed in batches.
+
+Depending on the complexity of your content model, lowering the batch size might speed up these operations.