Lua API docs: Add warning that schematic placing is cached

doc/lua_api.txt

@@ -3229,6 +3229,10 @@ These functions return the leftover itemstack.
    * `force_placement` is a boolean indicating whether nodes other than `air` and
      `ignore` are replaced by the schematic
    * Returns nil if the schematic could not be loaded.
    * **Warning**: Once you have loaded a schematic from a file, it will be cached. Future calls
      will always use the cached version and the replacement list defined for it,
      regardless of whether the file or the replacement list parameter have changed.
      The only way to load the file anew is to restart the server.

* `minetest.place_schematic_on_vmanip(vmanip, pos, schematic, rotation, replacement, force_placement)`:
    * This function is analogous to minetest.place_schematic, but places a schematic onto the