Better doc reference of CachingConfigurer

Update the documentation to better reflect the fact that a @EnableCaching
configuration class must implement CachingConfigurer in order to provide
advanced customizations to the cache abstraction.

Author: snicoll

src/asciidoc/index.adoc

@@ -49628,6 +49628,13 @@ options to be specified that influence the way the caching behavior is added to
 application through AOP. The configuration is intentionally similar with that of
 <<tx-annotation-driven-settings, `@Transactional`>>:
 
+[NOTE]
+====
+Advanced customizations using Java config require to implement `CachingConfigurer`, refer
+to {javadoc-baseurl}/org/springframework/cache/annotation/CachingConfigurer.html[the
+javadoc for more details].
+====
+
 [[cache-annotation-driven-settings]]
 .Cache annotation settings
 [cols="1,1,1,3"]