Improve documentation for liquid_alternative_* (minetest#12810)

doc/lua_api.txt

@@ -1189,12 +1189,14 @@ Look for examples in `games/devtest` or `games/minetest_game`.
  * The cubic source node for a liquid.
  * Faces bordering to the same node are never rendered.
  * Connects to node specified in `liquid_alternative_flowing`.
+ * You *must* set `liquid_alternative_source` to the node's own name.
  * Use `backface_culling = false` for the tiles you want to make
  visible when inside the node.
 * `flowingliquid`
  * The flowing version of a liquid, appears with various heights and slopes.
  * Faces bordering to the same node are never rendered.
  * Connects to node specified in `liquid_alternative_source`.
+ * You *must* set `liquid_alternative_flowing` to the node's own name.
  * Node textures are defined with `special_tiles` where the first tile
  is for the top and bottom faces and the second tile is for the side
  faces.
@@ -8233,14 +8235,31 @@ Used by `minetest.register_node`.
  -- around it until `liquid_range` is reached;
  -- will drain out without a source;
  -- recommended drawtype: "flowingliquid".
- -- If it's "source" or "flowing" and `liquid_range > 0`, then
- -- both `liquid_alternative_*` fields must be specified
+ -- If it's "source" or "flowing", then the
+ -- `liquid_alternative_*` fields _must_ be specified
 
  liquid_alternative_flowing = "",
- -- Node that represents the flowing version of the liquid
-
  liquid_alternative_source = "",
- -- Node that represents the source version of the liquid
+ -- These fields may contain node names that represent the
+ -- flowing version (`liquid_alternative_flowing`) and
+ -- source version (`liquid_alternative_source`) of a liquid.
+ --
+ -- Specifically, these fields are required if any of these is true:
+ -- * `liquidtype ~= "none" or
+ -- * `drawtype == "liquid" or
+ -- * `drawtype == "flowingliquid"
+ --
+ -- Liquids consist of up to two nodes: source and flowing.
+ --
+ -- There are two ways to define a liquid:
+ -- 1) Source node and flowing node. This requires both fields to be
+ -- specified for both nodes.
+ -- 2) Standalone source node (cannot flow). `liquid_alternative_source`
+ -- must be specified and `liquid_range` must be set to 0.
+ --
+ -- Example:
+ -- liquid_alternative_flowing = "example:water_flowing",
+ -- liquid_alternative_source = "example:water_source",
 
  liquid_viscosity = 0,
  -- Controls speed at which the liquid spreads/flows (max. 7).