Explain in spawn pattern documentation that you can't have converging routes

[erikquinn]

In many approach controls, there are two different routes or STARs aircraft can enter a given airspace on, which the center provides spacing for "as one". But the sim is not "smart" enough to be able to separate like this-- it only provides longitudinal separation, at the time on spawn, on each given route. So if two routes converge before or right near the approach control's airspace boundary, they will hit each other without the user being able to prevent it.

The interim solution to this lack of capability has been to outlaw the concept of spawning aircraft on multiple routes like this-- if they converge, the airport designer must pick one and use it exclusively. If they do not, the spawn logic intentionally spawns one aircraft on every spawn pattern right by the airspace boundary, in order to FORCE these situations to be immediately apparent, so the designer can see the issue and correct it.

However, this is not clearly laid out in the documentation as a mandatory design principle of spawn patterns, and so we keep getting issue reports and questions and such, which would all be avoided with better documentation. So let's put the info in the docs for spawn patterns, with some sort of emphasis (bolded or otherwise).