[Security] document current user attribute in the main security doc entry

[94noni]

I know its documented rapidly (via a code diff example) on top of this file but here on a controller I thinks its more usefull rather than using the getUser of the abstract controller wdyt?

also unrelated but, would it be possible to display the class name here ? seeing only getUser can be misleading, what about Security::getUser because getUser is globally generic in the codebase

[94noni]

@javiereguiluz friendly ping, it this PR still worth it or may I close ? thx