Add information about Cloudflare limits to instance_limits.md

The page https://docs.gitlab.com/ee/administration/instance_limits is
intended to be a comprehensive list of all application and rate limits
which apply to GitLab.  Information about Cloudflare CDN rate limits and
DDoS protection was previously not included.  This MR adds documentation
about those limits to this page.

doc/administration/instance_limits.md

@@ -947,3 +947,7 @@ varies by file type:
 If a branch is merged while open merge requests still point to it, GitLab can
 retarget merge requests pointing to the now-merged branch. To learn more, read
 [Branch retargeting on merge](../user/project/merge_requests/getting_started.md#branch-retargeting-on-merge).
+
+## CDN-based limits on GitLab.com
+
+In addition to application-based limits, GitLab.com is configured to use Cloudflare's standard DDoS protection and Spectrum to protect Git over SSH. Cloudflare terminates client TLS connections but is not application aware and cannot be used for limits tied to users or groups. Cloudflare page rules and rate limits are configured with Terraform. These configurations are [not public](https://about.gitlab.com/handbook/communication/#not-public) because they include security and abuse implementations that detect malicious activities and making them public would undermine those operations.