clarify what the SITE_URL is for (tpo/tpa/ci-templates#1)

... and it's not for nothing! :)

service/static-shim.md

@@ -427,6 +427,12 @@ to only a *specific* site update, with a wrapper that will call
 `rsync` but limited to a specific directory. We also allow connections
 only from GitLab over SSH.
 
+This implies that the `SITE_URL` provided by the GitLab CI job over
+SSH, whether it is for the `rsync` or `static-update-component`
+commands, is actually ignored by the backend. It is used in the job
+definition solely to avoid doing two deploys in parallel to the same
+site, through the GitLab [`resource_group`](https://docs.gitlab.com/ee/ci/resource_groups/) mechanism.
+
 The public part of that key should be set in the GitLab project, as a
 File variable called `STATIC_GITLAB_SHIM_SSH_PRIVATE_KEY`. This way
 the GitLab runners get access to the private key and can deploy those