DOCSP-20402 Add support for copy existing to set allow disk use flag (#217)

* DOCSP-20402 Add support for copy existing to set allow disk use flag

Author: mongodbchez

source/source-connector/configuration-properties/copy-existing.txt

@@ -121,5 +121,13 @@ Settings
        | **Default**: ``16000``
        | **Accepted Values**: An integer
 
-.. _source-configuration-copy-existing-table-end:
+   * - | **copy.existing.allow.disk.use**
+     - | **Type:** boolean
+       |
+       | **Description:**
+       | When set to ``true``, 
+         the connector sets the copy existing aggregation 
+         to use temporary disk storage. 
+       | **Default**: ``true``
 
+.. _source-configuration-copy-existing-table-end: