doc: update sum type docs (#5702)

examples/lander.v

@@ -0,0 +1,60 @@
+// Example of sum types
+// Models a landing craft leaving orbit and landing on a world
+import rand
+import time
+
+struct Moon {
+}
+
+struct Mars {
+}
+fn (m Mars) dust_storm() bool {
+	return rand.int() >= 0
+}
+
+struct Venus {
+}
+
+type World = Moon | Mars | Venus
+
+struct Lander {
+}
+fn (l Lander) deorbit() {
+	println('leaving orbit')
+}
+fn (l Lander) open_parachutes(n int) {
+	println('opening $n parachutes')
+}
+
+fn wait() {
+	println('waiting...')
+	time.sleep(1)
+}
+
+fn (l Lander) land(w World) {
+	if w is Mars {
+		m := w as Mars
+		for m.dust_storm() {
+			wait()
+		}
+	}
+	l.deorbit()
+	match w {
+		Moon {} // no atmosphere
+		Mars {
+			// light atmosphere
+			l.open_parachutes(3)
+		}
+		Venus {
+			// heavy atmosphere
+			l.open_parachutes(1)
+		}
+	}
+	println('landed')
+}
+
+fn main() {
+	l := Lander {}
+	l.land(Venus{})
+	l.land(Mars{})
+}