Allow multiple cave liquids in a biome definition (#8481)

This allows games to specify biome cave liquids and avoid the old
hardcoded behaviour, but preserves the ability to have multiple
cave liquids in one biome, such as lava and water.

When multiple cave liquids are defined by the biome definition,
make each entire cave use a randomly chosen liquid, instead of
every small cave segment using a randomly chosen liquid.

Plus an optimisation:
Don't place nodes if cave liquid is defined as 'air'

1 files changed, 8 insertions, 4 deletions

diff --git a/doc/lua_api.txt b/doc/lua_api.txt
index f72b87625..f29b7d439 100644
--- a/doc/lua_api.txt
+++ b/doc/lua_api.txt
@@ -6510,10 +6510,14 @@ Used by `minetest.register_biome`.
         depth_riverbed = 2,
         -- Node placed under river water and thickness of this layer
 
-        node_cave_liquid = "default:water_source",
-        -- Nodes placed as a blob of liquid in 50% of large caves.
-        -- If absent, cave liquids fall back to classic behaviour of lava or
-        -- water distributed according to a hardcoded 3D noise.
+        node_cave_liquid = "default:lava_source",
+        node_cave_liquid = {"default:water_source", "default:lava_source"},
+        -- Nodes placed inside 50% of the medium size caves.
+        -- Multiple nodes can be specified, each cave will use a randomly
+        -- chosen node from the list.
+        -- If this field is left out or 'nil', cave liquids fall back to
+        -- classic behaviour of lava and water distributed using 3D noise.
+        -- For no cave liquid, specify "air".
 
         node_dungeon = "default:cobble",
         -- Node used for primary dungeon structure.