[Security] Added documentation for IS_AUTHENTICATED

[artyuum]

I believe that this part of the doc is wrong or might be confusing. In order to check if the user is simply logged in, they are suggesting using the IS_AUTHENTICATED_FULLY attribute. But later below, we learn that this attribute is the "stronger" one and that it cannot be used in that context because:

This is similar to IS_AUTHENTICATED_REMEMBERED, but stronger. Users who are logged in only because of a "remember me cookie" will have IS_AUTHENTICATED_REMEMBERED but will not have IS_AUTHENTICATED_FULLY.

So the appropriate attribute should have been IS_AUTHENTICATED_REMEMBERED:

All logged-in users have this, even if they are logged in because of a "remember me cookie". Even if you don't use the remember me functionality, you can use this to check if the user is logged in.

But unfortunately, its name is misleading and it took me some long minutes to understand it and why it has been named like that.

After digging into the AuthenticatedVoter for a better understanding of these attributes, I stumbled upon an undocumented attribute called IS_AUTHENTICATED.

I believe that this attribute should be present in the doc and should become the recommended one when checking if a user is simply logged in because its name is obviously clearer and better describes its intent.

I'm not sure why it's not documented but it has been brought by this symfony/symfony@e3aca7f.

[javiereguiluz]

@artyuum thanks for improving the docs!