Manual AI seats for matches: host toggles empty seats, co-op allies fight heroes

docs/MULTIPLAYER.md

@@ -6,8 +6,9 @@ Multiplayer is only available for users of a server (not in the standalone file)
 
 - Modes: **versus** (every player a master with an own heart), **coop** (every player an own master,
   together against AI masters and/or the humans) and a **coop campaign of 20 missions**.
-- 2 to 4 players per match; free slots can be AI masters.
+- 1 to 4 players per match; free slots are empty unless the host marks them as AI with `set_ai` before the start.
 - The match **pauses while a player is missing** (connection lost); a player who **leaves** is replaced by an AI.
+- Before the start, the host manually marks a free seat as AI with `{"t":"set_ai","side":n,"ai":true}` and clears it again with `ai:false`; side 0 stays human.
 - The humans (hero waves of the Order) as a third party (can be switched off), chat, alliances/teams.
 - Coop is "separate but helpful": own heart, gold, mana, research and creatures; allies never fight each
   other, their creatures help each other, gold can be sent, allied ground can be walked; a fallen heart only