I just discovered that we haven't any docs about synthetic services.

Synthetic services are services which are injected later on. For instance, the kernel service is synthetic, because it's injected later on:

<service id="kernel" synthetic="true" />

class Kernel
{
// ...
   pubic function initializeContainer()
   {
       // ...
       $container->set('kernel', $this);
   }
}

It should get documented in an article under components/dependency_injection