Explain auto docu and fix NewFlowPostgres

[ostcar]

@bastianjoel The bug with the auto generation of the environment happens, because I never documented, how the environment system works.

I added some explanation, how the system was designed. I also changed the NewFlowPostgres function, to it behaves correctly.

You can find the necessary changes for a service here for the autoupdate: OpenSlides/openslides-autoupdate-service#1426

If you are ok with this change, then we have to fix the other go services as well.

If you don't like it, we have to discuss, how we solve it in another way. The current implementation breaks the idea behind the environment package.

Another way would be, if the Postgres-flow gets an "init" method that has to be called after NewFlowPostgres.

postgres, err := NewFlowPostgres(lookup)
service := func(ctx conext.Context) error{
  postgres.init(ctx)
}

I like the returned init function better, because it makes it very hard to forget to call it. But an separate init method would be ok for me.