Give subgames the ability to disallow specific mapgens (#6792)

doc/lua_api.txt

@@ -53,7 +53,7 @@ Games are looked up from:
 
 where `gameid` is unique to each game.
 
-The game directory contains the file `game.conf`, which contains these fields:
+The game directory contains the file `game.conf`, which contains:
 
     name = <Human-readable full name of the game>
 
@@ -61,6 +61,16 @@ e.g.
 
     name = Minetest
 
+Optionally, game.conf can also contain:
+
+    disallowed_mapgens = <comma-separated mapgens>
+
+e.g.
+
+    disallowed_mapgens = v5,v6,flat
+
+These mapgens are removed from the list of mapgens for the game.
+
 The game directory can contain the file minetest.conf, which will be used
 to set default settings when running the particular game.
 It can also contain a settingtypes.txt in the same format as the one in builtin.