(DOCSP-13704): Added warning about the first TTL index. (#3728)

* (DOCSP-13704): Added warning about the first TTL index.

* (DOCSP-13704): Incorporated Eric's feedback.

Author: corryroot

source/core/index-ttl.txt

@@ -31,6 +31,14 @@ finite amount of time.
 Create a TTL Index
 ------------------
 
+.. warning::
+
+   After you create a TTL index, it might have a very large number of 
+   qualifying documents to delete at once. This large workload might 
+   cause performance issues on the server. To avoid these issues, plan 
+   to create the index during off hours, or delete qualifying documents 
+   in batches before you create the index for future documents.
+
 To create a TTL index, use the :method:`~db.collection.createIndex()`
 method on a field whose value is either a :ref:`date
 <document-bson-type-date>` or an array that contains date values, then specify the ``expireAfterSeconds`` option with the desired TTL value in