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.