Improve Documentation of Authentication Service Plugin API

## Gentics Mesh Version, operating system, or hardware.
- v0.39.2

The documentation for the Group/Role Filter-Handling for AuthenticationPlugins could be improved a little bit.

It's a little bit confusing, if the group now get removed when the filter returns true or when he return false?

https://github.com/gentics/mesh-plugin-examples/blob/master/authentication-plugin/src/main/java/com/gentics/mesh/plugin/AuthenticationExamplePlugin.java#L68

Maybe handle both cases in the Example Plugin and/or mention that in the configuration