mention that untagged jobs will always run on amd64

service/ci.md

@@ -610,6 +610,10 @@ 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
 your job requires a lot of memory.
 
+If your job requires the `amd64` architecture, specifying this tag by itself is
+redundant because only runners with this architecture are configured to run
+untagged jobs. Jobs without any tags will only run on `amd64` runners.
+
 ### Upstream release schedules
 
 GitLab CI is an integral part of GitLab itself and gets released along