docs(kms): add an example of a key lookup alias (#23663)

This has confused a customer.


----

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

Author: rix0rrr

packages/@aws-cdk/aws-kms/lib/key-lookup.ts

@@ -4,6 +4,8 @@
 export interface KeyLookupOptions {
   /**
    * The alias name of the Key
+   *
+   * Must be in the format `alias/<AliasName>`.
    */
   readonly aliasName: string;
 }