Document seldom-used trigger functionality

Added a define for use in overworld triggers where the goal is to run the provided script in the immediate script context instead of the global one like normal. This is used in triggers in the desert, on faraway island, and a couple other places to do weather effects quickly and on the same frame. Suggestions for a better define name are welcome.

src/field_control_avatar.c

@@ -884,7 +884,7 @@ static u8 *TryRunCoordEventScript(struct CoordEvent *coordEvent)
             DoCoordEventWeather(coordEvent->trigger);
             return NULL;
         }
-        if (coordEvent->trigger == 0)
+        if (coordEvent->trigger == VAR_RUN_TRIGGER_IMMEDIATELY)
         {
             RunScriptImmediately(coordEvent->script);
             return NULL;