docs(trip): add Air Canada ref ADSPID for return leg

bookings/flights-ms.md

@@ -3,9 +3,12 @@
 LK flies with MS on both legs (YVR ↔ SFO). She leaves the group Sep 22
 and rejoins Sep 27 around 5 PM in LA.
 
-✅ **Booked Aug 31, 2026 — United confirmation `OMK75H`**
+✅ **Booked Aug 31, 2026 — United confirmation `OMK75H` · Air Canada ref `ADSPID`**
 ([eTicket PDF](United_Flights_YVR-SFO_OMK75H_eTicket.pdf))
 
+Use `OMK75H` on united.com (trip view, outbound check-in); use `ADSPID` on
+aircanada.com for the return leg — bag purchase and seat selection.
+
 ## Vancouver → San Francisco
 
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