Resolve "docs add more curl usage examples"

doc/api/README.md

@@ -72,9 +72,11 @@ Only API version v4 is available. Version v3 was removed in
 
 API requests should be prefixed with both `api` and the API version. The API
 version is defined in [`lib/api.rb`](https://gitlab.com/gitlab-org/gitlab/tree/master/lib/api/api.rb).
-For example, the root of the v4 API is at `/api/v4`.
+For example, the root of the v4 API is at `/api/v4`. The following sections illustrate different uses:
 
-Example of a valid API request using cURL:
+### Valid API request
+
+If you have a GitLab instance at `gitlab.example.com`:
 
 ```shell
 curl "https://gitlab.example.com/api/v4/projects"
@@ -83,6 +85,29 @@ curl "https://gitlab.example.com/api/v4/projects"
 The API uses JSON to serialize data. You don't need to specify `.json` at the
 end of an API URL.
 
+### API request to expose HTTP response headers
+
+If you want to expose HTTP response headers, use the `--include` option:
+
+```shell
+curl --include "https://gitlab.example.com/api/v4/projects"
+HTTP/2 200
+...
+```
+
+This can help you investigate an unexpected response.
+
+### API Request that includes the exit code
+
+If you want to expose the HTTP exit code, include the `--fail` option:
+
+```shell script
+curl --fail "https://gitlab.example.com/api/v4/does-not-exist"
+curl: (22) The requested URL returned error: 404
+```
+
+The HTTP exit code can help you diagnose the success or failure of your REST call.
+  
 ## Authentication
 
 Most API requests require authentication, or will return public data only when