Clarify Spring Security concepts/annotations and how they relate to Vaadin

[bennewi]

https://github.com/vaadin/docs/blob/latest/articles/security/enabling-security.adoc

This page assumes too much advanced knowledge of both javax security and Spring Security. In particular the introduction of annotations AnonymousAllowed, PermitAll, RolesAllowed, and DenyAll is unhelpful if you don't know that AnonymousAllowed is a Vaadin class and the PermitAll, RolesAllowed, and DenyAll are javax.security classes.

Please make this explanation understandable for people who are unfamiliar with Spring Security and javax.annotations and how these are related to Vaadin