It is on you to detect that the help flag has been set and then (probably) print get_help_string.
You can modify the help option using the help_option field in the return value of the file.

Register more types converters using the register_type function.
There are already type converters registered for "string", "number" and "boolean".

Finally use parse to parse a string array using a given config.

It can be used to only parse a part of the string array.
To tell it where to start parsing in the array provide the start_index (third param).
To know where it stopped parsing use the second return value.
Both of these indexes are one based including including.

Here is a config i used during development:

local result = parse(arg, {
  options = {
    {
      field = "option", -- results in a boolean, true or false, set or not set
      long = "option",
      short = "o",
      description = "my option",
      flag = true,
    },
    {
      field = "foo", -- results in an array with 1 or 2 numbers
      long = "foo",
      short = "f",
      min_params = 1,
      max_params = 2,
      type = "number",
    },
    {
      field = "bar", -- results in a single value, a string
      short = "b",
      single_param = true,
      type = "string",
    },
    {
      field = "baz", -- results in an array with 3 strings
      long = "baz",
      params = 3,
      type = "string",
      optional = true,
    },
  },
  positional = {
    {
      name = "yes",
      field = "yes", -- results in a boolean value
      description = "a bool, yes",
      type = "boolean", -- the first arg that is not consumed by options must be a bool
      single = true, -- and it must be provided
      default_value = false, -- and if not provided it's not an error, it's just false
    },
    {
      name = "files",
      field = "files",
      type = "string",
      min_amount = 0,  -- all other positional args
    }
  },
})

And here's an example for how i use it in general at the moment

local arg_parser = require("arg_parser")
local args_config = {
  -- configure options and positional args here
}

local args
do
  local err
  args, err = arg_parser.parse({...}, args_config)
  if (not args) or args.help then
    if not args then
      print(err)
      print()
    end
    print(arg_parser.get_help_string(args_config))
    return
  end
end

There are sumneko.lua EmmyLua docs at the bottom of the arg_parser.lua file, read those to understand how to configure options.

-- TODO: spend some time making the readme better.