Add API documentation

doc/api/lint.md

@@ -4,11 +4,14 @@ group: Continuous Integration
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 ---
 
-# Validate the `.gitlab-ci.yml` (API)
+# CI Lint API
+
+## Validate the CI YAML config
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/5953) in GitLab 8.12.
 
-Checks if your `.gitlab-ci.yml` file is valid.
+Checks if CI/CD YAML configuration is valid. This endpoint validates basic CI/CD
+configuration syntax. It doesn't have any namespace specific context.
 
 ```plaintext
 POST /ci/lint
@@ -16,13 +19,15 @@ POST /ci/lint
 
 | Attribute  | Type    | Required | Description |
 | ---------- | ------- | -------- | -------- |
-| `content`  | string    | yes      | the `.gitlab-ci.yaml` content|
+| `content`              | string     | yes      | The CI/CD configuration content. |
+| `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\"]}}"}'
 ```
 
-Be sure to copy paste the exact contents of `.gitlab-ci.yml` as YAML is very picky about indentation and spaces.
+Be sure to paste the exact contents of your GitLab CI/CD YAML config because YAML
+is very sensitive about indentation and spacing.
 
 Example responses:
 
@@ -53,3 +58,39 @@ Example responses:
     "error": "content is missing"
   }
   ```
+
+### YAML expansion
+
+The expansion only works for CI configurations that don't have local [includes](../ci/yaml/README.md#include).
+
+Example contents of a `.gitlab-ci.yml` passed to the CI Lint API with
+`include_merged_yaml` set as true:
+
+```yaml
+include:
+  remote: 'https://example.com/remote.yaml'
+
+test:
+  stage: test
+  script:
+    - echo 1
+```
+
+Example contents of `https://example.com/remote.yaml`:
+
+```yaml
+another_test:
+  stage: test
+  script:
+    - echo 2
+```
+
+Example response:
+
+```json
+{
+  "status": "valid",
+  "errors": [],
+  "merged_config": "---\n:another_test:\n  :stage: test\n  :script: echo 2\n:test:\n  :stage: test\n  :script: echo 1\n"
+}
+```