doc: fix enforce option handling

guides/plugins/doc_fields.md

@@ -8,7 +8,7 @@ accessible information.
 Below is the final code generated by the plugin, which includes comprehensive
 documentation for the `User` struct:
 
-```
+```elixir
 defmodule User do
   @typedoc """
   @type t(age) :: %User{age: age | nil, name: String.t() | nil}
@@ -33,6 +33,7 @@ end
 ```
 
 ## Implement
+
 ```elixir
 defmodule Guides.Plugins.DocFields do
   @moduledoc """
@@ -115,11 +116,12 @@ defmodule Guides.Plugins.DocFields do
     fields =
       Enum.map(definition.fields, fn field ->
         name = Keyword.fetch!(field, :name)
-
         type = Keyword.fetch!(field, :type)
 
+        enforce = Keyword.get(definition.options, :enforce, false)
+
         type =
-          if Keyword.get(field, :enforce, false) or Keyword.has_key?(field, :default) do
+          if Keyword.get(field, :enforce, enforce) or Keyword.has_key?(field, :default) do
             Macro.to_string(type)
           else
             # escape `|`
@@ -164,6 +166,7 @@ end
 ```
 
 ## Usage
+
 ```elixir
 defmodule User do
   @moduledoc false