document new TPA runner tag (see tpo/tpa/gitlab#110)

service/ci.md

@@ -344,6 +344,7 @@ We currently use the following tags:
    explicitly be able to run `DinD`
  * `interactive web terminal`: supports [interactively debugging
    jobs](https://docs.gitlab.com/ee/ci/interactive_web_terminal/)
+ * `TPA`: runners managed by the sysadmin team
  * `fdroid`: provided as a courtesy by the [F-Droid project](https://f-droid.org/)
 
 Use tags in your configuration only if your job can be fulfilled by