Improve Lua API documentation
Changes:
* Add extra documentation for remove_item.
This isn't as silly as it sounds: iterate a list containing items with
unique metadata each, and remove_item the first one you find, placing
into a different - fine, except the remove will invariably remove the
wrong one, leading to items being duplicated and others destroyed.
Arguably it's a bug, and Inventory::removeItem should actually remove
the item you tell it to. (i.e. if the passed ItemStack has metadata,
match it). But it seems reasonable to just document the behaviour
more clearly.
* Improve string_to_pos documentation.
* Clarify dig_node documentation (return value).
* Better on_step documentation.
* get_nodemeta -> get_meta.
* Other minor fixes.