Add clarification on when @DevaultValue is used during binding

Closes gh-26742

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/DefaultValue.java

@@ -25,7 +25,13 @@ import java.lang.annotation.Target;
 /**
  * Annotation that can be used to specify the default value when binding to an immutable
  * property. This annotation can also be used with nested properties to indicate that a
- * value should always be bound (rather than binding {@code null}).
+ * value should always be bound (rather than binding {@code null}). The value from this
+ * annotation will only be used if the property is not found in the property sources used
+ * by the {@link Binder}. For example, if the property is present in the
+ * {@link org.springframework.core.env.Environment} when binding to
+ * {@link org.springframework.boot.context.properties.ConfigurationProperties @ConfigurationProperties},
+ * the default value for the property will not be used even if the property value is
+ * empty.
  *
  * @author Madhura Bhave
  * @since 2.2.0