Document default typing requirements in GenericJackson2JsonRedisSerializer

[snicoll]

See https://github.com/spring-projects/spring-boot/issues/27577

Looking at both the javadoc of the class and the reference guide, I haven't found a place where the need for enabling that feature on a custom ObjectMapper is documented. Given that the exception we get back is quite cryptic, it would be nice to have a dedicated section in the reference guide where this class is referenced.