Document schema namespace search order

[TethysSvensson]

Suppose you have the following three files:

# In events.fbs
namespace Events;
enum Event { HOUSE_ON_FIRE, ... }

# In kitchen_events.fbs
namespace Kitchen.Events;
enum Event { OUT_OF_BUTTER, ... }

# In kitchen.fbs

include "events.fbs";
include "kitchen_events.fbs";

namespace Kitchen;

table KitchenState {
    eventlog: [Events.Event];
}

We need to document which of the two enums the eventlog refer to and whether we do the same thing as upstream.