[Fix #1119] Charles comments

Signed-off-by: fjtirado <ftirados@redhat.com>

Author: fjtirado

dsl-reference.md

@@ -2297,7 +2297,7 @@ timeout:
 
 ### Duration
 
-Defines a duration.
+Defines a duration. Durations can be defined through properties, with an ISO 8601 string or with a runtime expression that is evaluated to an ISO 8601 string
 
 #### Properties