docs(cloudwatch): Both defaultInterval and start for dashboard ca…

…nnot be set at once (#26971)

This PR adds comments that both defaultInterval and start for dashboard cannot be set at once.

The following is a validation code for them.

https://github.com/go-to-k/aws-cdk/blob/05d1c524e356f0e3cbd4799c6f7d233a44301954/packages/aws-cdk-lib/aws-cloudwatch/lib/dashboard.ts#L135-L137

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk-lib/aws-cloudwatch/lib/dashboard.ts

@@ -37,9 +37,11 @@ export interface DashboardProps {
    * Interval duration for metrics.
    * You can specify defaultInterval with the relative time(eg. cdk.Duration.days(7)).
    *
+   * Both properties `defaultInterval` and `start` cannot be set at once.
+   *
    * @default When the dashboard loads, the defaultInterval time will be the default time range.
    */
-  readonly defaultInterval?: Duration
+  readonly defaultInterval?: Duration;
 
   /**
    * The start of the time range to use for each widget on the dashboard.
@@ -49,6 +51,8 @@ export interface DashboardProps {
    * You can also use start along with an end field, to specify an absolute time range.
    * When specifying an absolute time range, use the ISO 8601 format. For example, 2018-12-17T06:00:00.000Z.
    *
+   * Both properties `defaultInterval` and `start` cannot be set at once.
+   *
    * @default When the dashboard loads, the start time will be the default time range.
    */
   readonly start?: string;