Add Geo proxying docs when primary is down

doc/administration/geo/secondary_proxy/index.md

@@ -140,6 +140,18 @@ for details.
   To use TLS certificates with Let's Encrypt, you can manually point the domain to one of the Geo sites, generate
   the certificate, then copy it to all other sites.
 
+## Behavior of secondary sites when the primary Geo site is down
+
+Considering that web traffic is proxied to the primary, the behavior of the secondary sites differs when the primary
+site is inaccessible:
+
+- UI and API traffic return the same errors as the primary (or fail if the primary is not accessible at all), since they are proxied.
+- For repositories that already exist on the specific secondary site being accessed, Git read operations still work as expected,
+  including authentication through HTTP(s) or SSH.
+- Git operations for repositories that are not replicated to the secondary site return the same errors
+  as the primary site, since they are proxied.
+- All Git write operations return the same errors as the primary site, since they are proxied.
+
 ## Features accelerated by secondary Geo sites
 
 Most HTTP traffic sent to a secondary Geo site can be proxied to the primary Geo site. With this architecture,