Dungeongen: Remove most hardcoded dungeon nodes (#8594)

Biome-defined dungeon nodes was added as a feature to MT 5.0.0.
So now remove most of the hardcoded dungeon node code that assumes a
game has stone, sandstone, desert stone, and no other stone types.
If biome-defined dungeon nodes are not found, dungeon nodes fall back
to the 'cobble' mapgen alias if present, if not present they fall back
to biome-defined 'stone'.
Remove now-unnecessary mapgen aliases from MapgenBasic. Non-mgv6 games
now only need to define 3 to 5 mapgen aliases.

Document dungeon parameters.

Make c_lava_source fallback to c_water_source as both are used as cave
liquids.

doc/lua_api.txt

@@ -282,36 +282,26 @@ 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:
+of the game's nodes are to be used for core mapgen generation. 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_river_water_source` is required for mapgens with sloping rivers where
+it is necessary to have a river liquid node with a short `liquid_range` and
+`liquid_renewable = false` to avoid flooding.
 
-Not required if cave liquid nodes are set in biome definitions.
+#### Deprecated aliases
 
 * mapgen_lava_source
-
-#### Dungeons
-
-Not required if dungeon nodes are set in biome definitions.
-
 * mapgen_cobble
-* mapgen_stair_cobble
-* mapgen_mossycobble
-* mapgen_desert_stone
-* mapgen_stair_desert_stone
-* mapgen_sandstone
-* mapgen_sandstonebrick
-* mapgen_stair_sandstone_block
+
+Use biome-defined cave liquids and dungeon nodes instead.
 
 ### Aliases needed for Mapgen V6