Add HTML comment explaining 503 issue

We won't be able to get around this URL presenting a false positive
to the link checker. What we CAN do, though, is put in a HTML
comment to save frustration, month after month.

doc/administration/git_protocol.md

@@ -120,5 +120,7 @@ production environment, you can use the following Prometheus query:
 sum(rate(gitaly_git_protocol_requests_total[1m])) by (grpc_method,git_protocol,grpc_service)
 ```
 
+<!-- This link sporadically returns a 503 during automated link checking but is correct -->
+
 You can view what Git protocol versions are being used on GitLab.com at
 <https://dashboards.gitlab.com/d/pqlQq0xik/git-protocol-versions>.