TaskExecution documentation should describe what happens when multiple Executor beans are present

In the [reference guide](https://docs.spring.io/spring-boot/3.3/reference/features/task-execution-and-scheduling.html) we state that GraphQL would use an `Executor` if it is present. But it would also use the one named `applicationTaskExecutor` in case of multiple matches. 

We should clarify that in the reference guide.