Added docs about default_lifetime

This will fix #14347

Author: Nyholm

reference/configuration/framework.rst

@@ -2847,9 +2847,14 @@ Can also be the service id of another cache pool where tags will be stored.
 default_lifetime
 """"""""""""""""
 
-**type**: ``integer``
+**type**: ``integer`` | ``string``
 
-Default lifetime of your cache items in seconds.
+Default lifetime of your cache items. Given an integer value to set the default
+lifetime in seconds. A string value could be ISO 8601 time interval, like ``"PT5M"``
+or a PHP date expression that is accepted by ``strtotime()``, like ``"5 minutes"``.
+
+If no value is provided, the cache adapter will fallback to the default value on
+the actual cache storage.
 
 provider
 """"""""