The cache abstraction is a core feature of the Spring Framework. Basic
features such as `@EnableCaching` are therefore available by default with
no extra dependencies necessary.

However, the actual cache adapters for JCache, Ehcache 2.x, Caffeine and
Guava are located in a separated module, `spring-context-support`. Spring
Boot provides that artifact via the `spring-boot-starter-cache` starter.

It is quite easy to "only" add the cache library dependencies and forget
about this extra dependencies since `@EnableCaching` is available by
default. This commit clarifies the role of the starer in each section so
that it is more obvious. We're already explaining this at the beginning
of the section but it seems that's not enough.

Closes gh-7071