Document that ConfigurationProperties can't use SpEL

Closes gh-1768

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -634,6 +634,9 @@ Nested POJO properties can also be created (so a setter is not mandatory) if the
 default constructor, or a constructor accepting a single value that can be coerced from
 String. Some people use Project Lombok to add getters and setters automatically.
 
+TIP: Contrary to `@Value`, SpEL expressions are not evaluated prior to injecting a value
+in the relevant `@ConfigurationProperties` bean as said value is usually externalized.
+
 The `@EnableConfigurationProperties` annotation is automatically applied to your project
 so that any beans annotated with `@ConfigurationProperties` will be configured from the
 `Environment` properties. This style of configuration works particularly well with the

spring-boot/src/main/java/org/springframework/boot/context/properties/ConfigurationProperties.java

@@ -26,6 +26,9 @@ import java.lang.annotation.Target;
  * Annotation for externalized configuration. Add this to a class definition or a
  * {@code @Bean} method in a {@code @Configuration} class if you want to bind and validate
  * some external Properties (e.g. from a .properties file).
+ * <p>
+ * Note that contrary to {@code @Value}, SpEL expressions are not evaluated since property
+ * values are externalized.
  *
  * @author Dave Syer
  * @see ConfigurationPropertiesBindingPostProcessor