Refine `@Contract` Javadoc to mention `this` and `new` return values

As documented in Jetbrains variant of `@Contract` [here](https://github.com/JetBrains/java-annotations/blob/master/src/jvmMain/java/org/jetbrains/annotations/Contract.java#L45-L48) and as suggested by @mp911de, `org.springframework.lang.Contract` should document `this` and `new` return values for consistency even if they are potentially less common use cases.