Documents and improves the variable names of the shuttle subsystem (#63946)

code/modules/shuttle/shuttle.dm

@@ -220,7 +220,7 @@
 	if(!port_destinations)
 		port_destinations = id
 
-	SSshuttle.stationary += src
+	SSshuttle.stationary_docking_ports += src
 
 /obj/docking_port/stationary/Initialize(mapload)
 	. = ..()
@@ -239,7 +239,7 @@
 
 /obj/docking_port/stationary/unregister()
 	. = ..()
-	SSshuttle.stationary -= src
+	SSshuttle.stationary_docking_ports -= src
 
 /obj/docking_port/stationary/Destroy(force)
 	if(force)
@@ -281,13 +281,13 @@
 
 /obj/docking_port/stationary/transit/Initialize(mapload)
 	. = ..()
-	SSshuttle.transit += src
+	SSshuttle.transit_docking_ports += src
 
 /obj/docking_port/stationary/transit/Destroy(force=FALSE)
 	if(force)
 		if(get_docked())
 			log_world("A transit dock was destroyed while something was docked to it.")
-		SSshuttle.transit -= src
+		SSshuttle.transit_docking_ports -= src
 		if(owner)
 			if(owner.assigned_transit == src)
 				owner.assigned_transit = null
@@ -385,11 +385,11 @@
 		else
 			SSshuttle.assoc_mobile[id] = 1
 
-	SSshuttle.mobile += src
+	SSshuttle.mobile_docking_ports += src
 
 /obj/docking_port/mobile/unregister()
 	. = ..()
-	SSshuttle.mobile -= src
+	SSshuttle.mobile_docking_ports -= src
 
 /obj/docking_port/mobile/Destroy(force)
 	if(force)