Make explicit, that some @Scheduled annotation attributes are in SpEL

[ullenboom]

request for a javadoc improvement.

some classes like https://docs.spring.io/spring-framework/docs/6.0.0-SNAPSHOT/javadoc-api/org/springframework/cache/annotation/Cacheable.html makes it clear that some attributes are SpEL:

on the other hand, a class like https://docs.spring.io/spring-framework/docs/6.0.0-SNAPSHOT/javadoc-api/org/springframework/scheduling/annotation/Scheduled.html makes no references to the spel:

the javadoc is identical. suggestion: some uniform description in the javadoc that some annotation attributes can hold a spel.