Add docs about the security user value resolver

[linaori]

In symfony/symfony#18510 I've added an ArgumentValueResolver to add the UserInterface to resolve to the logged-in user. However, this is not documented yet. If someone is up for it, feel free to write docs, otherwise I will write something soon (tm) when I have some time.

If I've started, I will refer this issue so it won't be done twice.

[wouterj]

ping @iltar, do you have some time to do this before the stable release?

[linaori]

I totally forget to mention it: #7060