Review slice documentation to clarify the scope and alternative options

[snicoll]

Looking at the doc of, for instance, @JdbcTest it is a bit vague what "pure JDBC-related tests" means. Someone may thing that anything Spring Boot has auto-configuration for related to a DataSource will work.

We should also have a section that provides some escape plans with warnings on non-supported case (i.e. importing manually the auto-configuration that should be applied).

At the moment there are interesting information in the Javadoc that would be worth moving to the ref guide as well.

cc @mminella