[Advanced DI] Short guide on how to make a service alias private

[linaori]

A colleague of mine was looking for a way to make an alias private due to the nature of how privates work, the original services are private too. He managed to trace back in the code that there's a secondary way of defining the Alias; By using Symfony\Component\DependencyInjection\Alias::__construct($id, $public = true).

An example can be found in the Doctrine bundle (DoctrineExtension.php): $container->setAlias($aliasId, new Alias($serviceId, false));. I think this would be a useful and easy feature to document.

[wouterj]

In order for this to be an easy pick, we have to show where to put this information.

For now, this can be added to components/dependency_injection/advanced.rst. But I'd like to revisit that article in the future (#4228).

[linaori]

I think that for now, it could just be a small paragraph below the current Alias and be taken along when the section is revised.

[javiereguiluz]

@iltar is this 3-year old issue still relevant after all change sin the DI component? Thanks.

[linaori]

Not entirely sure, but I don't see others asking for the feature so I suppose we can close it 👍