fix(elixir): resolve mix docs warnings in templates

- streaming.j2: add @moduledoc (was hidden)
- errors.j2: use Exception.t() instead of Splode macro types, drop UnknownError from typedoc

Amp-Thread-ID: https://ampcode.com/threads/T-019c390a-abe3-726d-a4a1-9cfc2522667b
Co-authored-by: Amp <amp@ampcode.com>

Author: mikehostetler

xdk-gen/templates/elixir/errors.j2

@@ -9,12 +9,13 @@ defmodule Xdk.Errors do
     ],
     unknown_error: Xdk.Errors.UnknownError
 
-  @type error ::
-          Xdk.Errors.ApiError.t()
-          | Xdk.Errors.RateLimitError.t()
-          | Xdk.Errors.TransportError.t()
-          | Xdk.Errors.DecodeError.t()
-          | Xdk.Errors.UnknownError.t()
+  @typedoc """
+  Any error returned by the XDK client.
+
+  One of `Xdk.Errors.ApiError`, `Xdk.Errors.RateLimitError`,
+  `Xdk.Errors.TransportError`, or `Xdk.Errors.DecodeError`.
+  """
+  @type error :: Exception.t()
 end
 
 defmodule Xdk.Errors.Api do

xdk-gen/templates/elixir/streaming.j2

@@ -1,6 +1,11 @@
 {% raw %}
 defmodule Xdk.Streaming do
-  @moduledoc false
+  @moduledoc """
+  Low-level NDJSON streaming helper for the X API.
+
+  Used internally by `Xdk.Stream` to consume newline-delimited JSON
+  streams (filtered stream, sample stream, etc.).
+  """
 
   @spec ndjson_stream(Xdk.t(), atom(), String.t(), keyword()) :: Enumerable.t()
   def ndjson_stream(%Xdk{} = client, method, path, opts \\ []) do