docs(events): mention property requirements when creating custom events (#29287)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

When creating an EventPattern for a custom event, a few properties are required. Documentation currently isn't clear on which properties are required. This change links to AWS documentation that describes the required properties for custom events.

### Description of changes

Added text stating that some optional properties are required for custom events and provided link to docs. 

### Description of how you validated changes

No validation

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: evzzk

Diff for: packages/aws-cdk-lib/aws-events/lib/event-pattern.ts

@@ -262,6 +262,9 @@ export class Match implements IResolvable {
  * - Number matching is at the string representation level. For example, 300,
  *   300.0, and 3.0e2 are not considered equal.
  *
+ * For custom events, some optional properties are required. For more information, see
+ * [Minimum information needed for a valid custom event](https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-events-structure.html#eb-custom-event).
+ *
  * @see https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/CloudWatchEventsandEventPatterns.html
  */
 export interface EventPattern {