Improve Javadoc for @value regarding supported expressions

Closes gh-25284

Author: sbrannen

spring-beans/src/main/java/org/springframework/beans/factory/annotation/Value.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -23,14 +23,17 @@
 import java.lang.annotation.Target;
 
 /**
- * Annotation at the field or method/constructor parameter level
- * that indicates a default value expression for the affected argument.
+ * Annotation used at the field or method/constructor parameter level
+ * that indicates a default value expression for the annotated element.
  *
- * <p>Typically used for expression-driven dependency injection. Also supported
- * for dynamic resolution of handler method parameters, e.g. in Spring MVC.
+ * <p>Typically used for expression-driven or property-driven dependency injection.
+ * Also supported for dynamic resolution of handler method arguments &mdash; for
+ * example, in Spring MVC.
  *
- * <p>A common use case is to assign default field values using
- * <code>#{systemProperties.myProp}</code> style expressions.
+ * <p>A common use case is to inject values using
+ * <code>#{systemProperties.myProp}</code> style SpEL (Spring Expression Language)
+ * expressions. Alternatively, values may be injected using
+ * <code>${my.app.myProp}</code> style property placeholders.
  *
  * <p>Note that actual processing of the {@code @Value} annotation is performed
  * by a {@link org.springframework.beans.factory.config.BeanPostProcessor
@@ -55,7 +58,8 @@
 public @interface Value {
 
 	/**
-	 * The actual value expression &mdash; for example, <code>#{systemProperties.myProp}</code>.
+	 * The actual value expression such as <code>#{systemProperties.myProp}</code>
+	 * or property placeholder such as <code>${my.app.myProp}</code>.
 	 */
 	String value();