Doc: warn about node above pos2 that must be generated

doc/lua_api.md

@@ -6086,8 +6086,11 @@ Environment access
     * Generate the `node_dust` of the biome (see [Biome definition]) within the
       VoxelManip `vm` and in the area from `pos1` to `pos2`.
     * `pos1` and `pos2` should have the same X and Z size than mapgen chunks.
+    * Nodes immediately above `pos2` must be generated (non-ignore). If the
+      mapgen does not over-generate 1 node up, consider offsetting `pos2.y`
+      by -1.
     * Reads the mapgen object `biomemap` (see [Mapgen Objects]), that must be
-      already computed (ie. using `minetest.generate_biomes`)
+      already computed (ie. using `minetest.generate_biomes`).
 * `minetest.clear_objects([options])`
     * Clear all objects in the environment
     * Takes an optional table as an argument with the field `mode`.