Document which @scheduled attributes support SpEL expressions

Closes gh-29290

(cherry picked from commit f8c8873)

spring-context/src/main/java/org/springframework/scheduling/annotation/Scheduled.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -117,9 +117,12 @@
 	 * last invocation and the start of the next.
 	 * <p>The time unit is milliseconds by default but can be overridden via
 	 * {@link #timeUnit}.
+	 * <p>This attribute variant supports Spring-style "${...}" placeholders
+	 * as well as SpEL expressions.
 	 * @return the delay as a String value &mdash; for example, a placeholder
 	 * or a {@link java.time.Duration#parse java.time.Duration} compliant value
 	 * @since 3.2.2
+	 * @see #fixedDelay()
 	 */
 	String fixedDelayString() default "";
 
@@ -135,9 +138,12 @@
 	 * Execute the annotated method with a fixed period between invocations.
 	 * <p>The time unit is milliseconds by default but can be overridden via
 	 * {@link #timeUnit}.
+	 * <p>This attribute variant supports Spring-style "${...}" placeholders
+	 * as well as SpEL expressions.
 	 * @return the period as a String value &mdash; for example, a placeholder
 	 * or a {@link java.time.Duration#parse java.time.Duration} compliant value
 	 * @since 3.2.2
+	 * @see #fixedRate()
 	 */
 	String fixedRateString() default "";
 
@@ -156,9 +162,12 @@
 	 * {@link #fixedRate} or {@link #fixedDelay} task.
 	 * <p>The time unit is milliseconds by default but can be overridden via
 	 * {@link #timeUnit}.
+	 * <p>This attribute variant supports Spring-style "${...}" placeholders
+	 * as well as SpEL expressions.
 	 * @return the initial delay as a String value &mdash; for example, a placeholder
 	 * or a {@link java.time.Duration#parse java.time.Duration} compliant value
 	 * @since 3.2.2
+	 * @see #initialDelay()
 	 */
 	String initialDelayString() default "";