Add notes in documentation to mention loginUser() and other methods from Symfony

[Alexis Lefebvre]

Is your feature request related to a problem? Please describe.
loginUser is available since Symfony 5.1, it can be used instead of some methods provided by this bundle: loginAs, loginClient and maybe others (will check later).

Acutally it is called by this bundle when it is available, see:

• GH-603

Dropping the support of Symfony 4.4 like in #631 means that loginUser will always be available to users.

Describe the solution you'd like
In the doc: https://github.com/liip/LiipFunctionalTestBundle/tree/master/doc

Add notes to explain when the native Symfony method can be called.

Describe alternatives you've considered
.

Additional context
I'm pretty sure that I already wrote something about the use of loginUser but I can't find it.

This GitHub notes format can be use, see:

• GH-635
• https://github.com/orgs/community/discussions/16925