Nevermind. I obviously need to improve my search skills. After further looking, I found this on Apple's support site:
Mark segments in your workout
If you want to mark a segment in your workout, double-tap the screen, then wait for the segment summary to appear on the screen. For example, in an Outdoor Run workout on a track, you could mark each lap or distance. In a 30-minute Indoor Cycle workout, you could split the session into three 10-minute segments.
To see all your segments after a workout:
1) Open the Activity app on your iPhone.
2) Tap the Workouts tab.
3) Tap the workout, then scroll down.
So apparently I must have tapped the watch face - probably when I was pushing back a sleeve to see it. Not sure if I'll use it, but it's always nice to discover new features.