DOCSP-39913 Add TTL Index Behavior Content (#7853) (#7884)

* DOCSP-39913 Add TTL Index Behavior Content

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Author: ianf-mongodb

source/core/index-ttl.txt

@@ -126,18 +126,39 @@ collection:
      }
    })
 
+.. important::
+
+   Consider the following before updating the ``expireAfterSeconds``
+   parameter of a TTL index:
+
+   - Changing the ``expireAfterSeconds`` parameter **does not** trigger a 
+     complete index rebuild. However, reducing the ``expireAfterSeconds`` 
+     value can make many documents eligible for immediate deletion, 
+     potentially causing performance issues due to the increased delete 
+     operations.
+
+   - The recommended approach is to manually delete documents in 
+     small batches before updating the TTL index. This helps control 
+     the impact on your cluster.
+
+   - Deleting many documents can fragment storage files, additionally 
+     impacting performance. You may need to run the 
+     :ref:`compact <compact>` command on your collection or 
+     perform a :ref:`replica-set-initial-sync` to reclaim space and 
+     optimize storage.
+
 Behavior
 --------
 
 Expiration of Data
 ~~~~~~~~~~~~~~~~~~
 
 TTL indexes expire documents after the specified number of seconds has
-passed since the indexed field value; i.e. the expiration threshold is
-the indexed field value plus the specified number of seconds.
+passed since the indexed field value. The expiration threshold is
+the indexed field value plus the specified number of seconds. 
 
 If the field is an array, and there are multiple date values in the
-index, MongoDB uses *lowest* (i.e. earliest) date value in the array to
+index, MongoDB uses *lowest* (earliest) date value in the array to
 calculate the expiration threshold.
 
 If the indexed field in a document is not a