DOCSP-14170 clarify TTL index requirements

Author: andf-mongodb

source/core/index-ttl.txt

@@ -29,14 +29,15 @@ of time or at a specific clock time. Data expiration is useful for certain types
 like machine generated event data, logs, and session information that
 only need to persist in a database for a finite amount of time.
 
-To create a TTL index, use the :method:`db.collection.createIndex()`
-method with the ``expireAfterSeconds`` option on a field whose value is
-either a :ref:`date <document-bson-type-date>` or an array that
-contains :ref:`date values <document-bson-type-date>`.
+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 :ref:`date values
+<document-bson-type-date>`, and specify the ``expireAfterSeconds``
+option with the desired TTL value in seconds.
 
 For example, to create a TTL index on the ``lastModifiedDate`` field of
-the ``eventlog`` collection, use the following operation in the
-:binary:`~bin.mongo` shell:
+the ``eventlog`` collection, with a TTL value of ``3600`` seconds, use
+the following operation in the :binary:`~bin.mongo` shell:
 
 .. code-block:: javascript