Adds note on authentication for /ci/lint

doc/api/lint.md

@@ -13,6 +13,8 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 Checks if CI/CD YAML configuration is valid. This endpoint validates basic CI/CD
 configuration syntax. It doesn't have any namespace specific context.
 
+Access to this endpoint requires authentication.
+
 ```plaintext
 POST /ci/lint
 ```
@@ -23,7 +25,7 @@ POST /ci/lint
 | `include_merged_yaml`  | boolean    | no       | If the [expanded CI/CD configuration](#yaml-expansion) should be included in the response. |
 
 ```shell
-curl --header "Content-Type: application/json" "https://gitlab.example.com/api/v4/ci/lint" --data '{"content": "{ \"image\": \"ruby:2.6\", \"services\": [\"postgres\"], \"before_script\": [\"bundle install\", \"bundle exec rake db:create\"], \"variables\": {\"DB_NAME\": \"postgres\"}, \"types\": [\"test\", \"deploy\", \"notify\"], \"rspec\": { \"script\": \"rake spec\", \"tags\": [\"ruby\", \"postgres\"], \"only\": [\"branches\"]}}"}'
+curl --header "Content-Type: application/json" --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/ci/lint" --data '{"content": "{ \"image\": \"ruby:2.6\", \"services\": [\"postgres\"], \"before_script\": [\"bundle install\", \"bundle exec rake db:create\"], \"variables\": {\"DB_NAME\": \"postgres\"}, \"types\": [\"test\", \"deploy\", \"notify\"], \"rspec\": { \"script\": \"rake spec\", \"tags\": [\"ruby\", \"postgres\"], \"only\": [\"branches\"]}}"}'
 ```
 
 Be sure to paste the exact contents of your GitLab CI/CD YAML configuration because YAML