Refer to Spring Boot implementation detection in docs

[eleftherias]

Summary

From the Spring Boot documentation:

If a single Spring Session module is present on the classpath, Spring Boot uses that store implementation automatically. If you have more than one implementation, you must choose the StoreType that you wish to use to store the sessions.

The Spring Session documentation does not mention that Spring Boot will automatically detect the implementation if there is a single module on the classpath.
We should add that to the "Spring Boot Configuration" section of the Spring Session documentation.