Lua_api.txt: Add documentation of required mapgen aliases

doc/lua_api.txt

@@ -214,7 +214,8 @@ when registering it.
 
 The `:` prefix can also be used for maintaining backwards compatibility.
 
-### Aliases
+Aliases
+-------
 Aliases can be added by using `minetest.register_alias(name, convert_to)` or
 `minetest.register_alias_force(name, convert_to)`.
 
@@ -235,6 +236,75 @@ you have an item called `epiclylongmodname:stuff`, you could do
 
 and be able to use `/giveme stuff`.
 
+Mapgen aliases
+--------------
+In a game, a certain number of these must be set to tell core mapgens which
+of the game's nodes are to be used by the core mapgens. For example:
+
+    minetest.register_alias("mapgen_stone", "default:stone")
+
+### Aliases needed for all mapgens except Mapgen v6
+
+Base terrain:
+
+"mapgen_stone"
+"mapgen_water_source"
+"mapgen_river_water_source"
+
+Caves:
+
+"mapgen_lava_source"
+
+Dungeons:
+
+Only needed for registered biomes where 'node_stone' is stone:
+"mapgen_cobble"
+"mapgen_stair_cobble"
+"mapgen_mossycobble"
+Only needed for registered biomes where 'node_stone' is desert stone:
+"mapgen_desert_stone"
+"mapgen_stair_desert_stone"
+Only needed for registered biomes where 'node_stone' is sandstone:
+"mapgen_sandstone"
+"mapgen_sandstonebrick"
+"mapgen_stair_sandstone_block"
+
+### Aliases needed for Mapgen v6
+
+Terrain and biomes:
+
+"mapgen_stone"
+"mapgen_water_source"
+"mapgen_lava_source"
+"mapgen_dirt"
+"mapgen_dirt_with_grass"
+"mapgen_sand"
+"mapgen_gravel"
+"mapgen_desert_stone"
+"mapgen_desert_sand"
+"mapgen_dirt_with_snow"
+"mapgen_snowblock"
+"mapgen_snow"
+"mapgen_ice"
+
+Flora:
+
+"mapgen_tree"
+"mapgen_leaves"
+"mapgen_apple"
+"mapgen_jungletree"
+"mapgen_jungleleaves"
+"mapgen_junglegrass"
+"mapgen_pine_tree"
+"mapgen_pine_needles"
+
+Dungeons:
+
+"mapgen_cobble"
+"mapgen_stair_cobble"
+"mapgen_mossycobble"
+"mapgen_stair_desert_stone"
+
 Textures
 --------
 Mods should generally prefix their textures with `modname_`, e.g. given
@@ -2417,6 +2487,8 @@ Call these functions only at load time!
 * `minetest.register_craftitem(name, item definition)`
 * `minetest.unregister_item(name)`
 * `minetest.register_alias(name, convert_to)`
+    * Also use this to set the 'mapgen aliases' needed in a game for the core
+    * mapgens. See 'Mapgen aliases' section above.
 * `minetest.register_alias_force(name, convert_to)`
 * `minetest.register_craft(recipe)`
     * Check recipe table syntax for different types below.