Commit 240f254 has introduced support for `java.util.Optional` in the
cache abstraction. If such type is present, the contained value is cached
if it is present.
This new feature slightly changed the semantic of `#result` that was
documented up till this commit as the "return value of the method
invocation". This is no longer true as `#result` for `Optional<T>`
refers to the `T` instance and not the `Optional` instance.
This commit clarifies both the javadoc and the documentation.
Issue: SPR-14587