Add description to id parameter of ec2 module

Add some details in the docs about how the id parameter of the ec2
module behaves.

library/cloud/ec2

@@ -31,6 +31,8 @@ options:
   id:
     description:
       - identifier for this instance or set of instances, so that the module will be idempotent with respect to EC2 instances.
+      - Note: this identifier is valid for at least 24 hours after the termination of the instance, and should not be reused for another call later on.
+      - For details, see the description of client token at U(http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Run_Instance_Idempotency.html).
     required: false
     default: null
     aliases: []