Add more info on how DAST works

doc/ci/examples/dast.md

 # Dynamic Application Security Testing with GitLab CI/CD
-This example shows how to run
 [Dynamic Application Security Testing (DAST)](https://en.wikipedia.org/wiki/Dynamic_program_analysis)
-on your project's source code by using GitLab CI/CD.
-DAST is using the popular open source tool
-[OWASP ZAProxy](https://github.com/zaproxy/zaproxy) to perform an analysis.
+is using the popular open source tool [OWASP ZAProxy](https://github.com/zaproxy/zaproxy)
+to perform an analysis on your running web application.
+It can be very useful combined with [Review Apps](../review_apps/index.md).
+
+## Example
 All you need is a GitLab Runner with the Docker executor (the shared Runners on
 GitLab.com will work fine). You can then add a new job to `.gitlab-ci.yml`,
@@ -14,22 +15,26 @@ called `dast`:
 ```yaml
 dast:
   image: owasp/zap2docker-stable
+  variables:
+    website: "https://example.com"
   script:
     - mkdir /zap/wrk/
-    - /zap/zap-baseline.py -J gl-dast-report.json -t https://example.com || true
+    - /zap/zap-baseline.py -J gl-dast-report.json -t $website || true
     - cp /zap/wrk/gl-dast-report.json .
   artifacts:
     paths: [gl-dast-report.json]
 ```
-The above example will create a `dast` job in your CI pipeline and will allow
-you to download and analyze the report artifact in JSON format.
+The above example will create a `dast` job in your CI/CD pipeline which will run
+the tests on the URL defined in the `website` variable (change it to use your
+own) and finally write the results in the `gl-dast-report.json` file. You can
+then download and analyze the report artifact in JSON format.
 TIP: **Tip:**
 Starting with [GitLab Enterprise Edition Ultimate][ee] 10.4, this information will
 be automatically extracted and shown right in the merge request widget. To do
 so, the CI job must be named `dast` and the artifact path must be
 `gl-dast-report.json`.
-[Learn more on dynamic application security testing results shown in merge requests](https://docs.gitlab.com/ee/user/project/merge_requests/dast.html).
+[Learn more about DAST results shown in merge requests](https://docs.gitlab.com/ee/user/project/merge_requests/dast.html).
 [ee]: https://about.gitlab.com/gitlab-ee/