update copy.existing table entry description

Author: Chris Cho

source/kafka-source.txt

@@ -184,7 +184,7 @@ an example source connector configuration file, see
 
    * - copy.existing
      - boolean
-     - | Copy existing data from all the collections being used as the source then add any changes after. It should be noted that the reading of all the data during the copy and then the subsequent change stream events may produce duplicated events. During the copy, clients can make changes to the data in MongoDB, which may be represented both by the copying process and the change stream. However, as the change stream events are idempotent the changes can be applied so that the data is eventually consistent. Renaming a collection during the copying process is not supported.
+     - | Copy existing data from source collections and convert them to Change Stream events on their respective topics. Any changes to the data that occur during the copy process are applied once the copy is completed.
        | **Default**: false
        | **Accepted Values**: ``true`` or ``false``