Masters for a match: the four sides red, green, blue and yellow (side 4 gets its own owner), and start_masters gives every master his core and his starting area

docs/MULTIPLAYER.md

@@ -58,5 +58,14 @@ per turn. Fog of war, particles and the camera are local and not part of the sha
 | E | Alliances and coop rules, AI masters for free and abandoned slots | planned |
 | F | Coop campaign: 20 missions with story (EN/DE) | planned |
 
-Still open on the way to C: player owners (today side 1 is the humans' and 2/3 the rivals' - four
-player sides need their own `Owner`s), the turn/turn-boundary client loop, and the state checksum.
+Still open on the way to C: the turn/turn-boundary client loop, the state checksum, and heroes and
+alliances for the new sides (today the hero gate path always leads to side 0's core, and 2/3 are the
+rivals' sides in free play).
+
+## Masters and sides
+
+A match plays on the four master sides `0, 2, 3, 4` (side 1 is the heroes'): red (side 0, `Owner::Player`),
+green (`Owner::Rival1`), blue (`Owner::Rival2`) and yellow (`Owner::Rival3`). `Game::start_masters(sides,
+local)` carves a 9x9 starting area and a 3x3 soul core for every master and remembers his economy in a
+`MasterState`; the places are worked out from the map size alone, so all clients agree without passing them
+over the network.