expand on why using runner name tags is a bad idea

service/ci.md

@@ -602,7 +602,9 @@ We currently use the following tags:
      numbers of processes (runners with >1TB memory)
    * `podman`: a `docker` executor which talks to the `podman` socket
      instead of Docker, might be better suited to build container images
- * **runner name**: for debugging purposes only!
+ * **runner name**: for debugging purposes only! allows pipelines to
+   target a specific runner, do not use as runners can come and go
+   without prior warning
 
 Use tags in your configuration only if your job can be fulfilled by
 only some of those runners. For example, only specify a memory tag if