chore(docs): clarify use of isOrganizationTrail

packages/@aws-cdk/aws-cloudtrail/README.md

@@ -190,7 +190,7 @@ trail.addLambdaEventSelector([ amazingFunction ]);
 ## Organization Trail
 
 It is possible to create a trail that will be applied to all accounts in an organization if the current account manages an organization.
-To enable this, the property `isOrganizationTrail` must be set. If this property is set and the current account does not manage an organization, the created trail will be created only for the account.
+To enable this, the property `isOrganizationTrail` must be set. If this property is set and the current account does not manage an organization, the stack will fail to deploy.
 
 ```ts
 new cloudtrail.Trail(this, 'OrganizationTrail', {

packages/@aws-cdk/aws-cloudtrail/lib/cloudtrail.ts

@@ -119,6 +119,8 @@ export interface TrailProps {
   /**
    * Specifies whether the trail is applied to all accounts in an organization in AWS Organizations, or only for the current AWS account.
    *
+   * If this is set to true then the current account _must_ be the management account. If it is not, then CloudFormation will throw an error.
+   *
    * If this is set to true and the current account is a management account for an organization in AWS Organizations, the trail will be created in all AWS accounts that belong to the organization.
    * If this is set to false, the trail will remain in the current AWS account but be deleted from all member accounts in the organization.
    *