docs/client-agent: clarify use of override_upstream_dns_servers (#5220)

* docs/client-agent: clarify use of override_upstream_dns_servers

The override_upstream_dns_servers option is recommended whenever
a user needs to override the DNS servers discovered from the system.
However, if a user supplies a DNS server that is only useable while
connected to a specific network, it will break the internet in
cases where it is not able to resolve these DNS servers.
Encourage users to supply both the VPN servers and the default
DNS servers.

* Update website/content/docs/api-clients/client-agent.mdx

Co-authored-by: Dan Heath <76443935+Dan-Heath@users.noreply.github.com>

* Update website/content/docs/api-clients/client-agent.mdx

Co-authored-by: Dan Heath <76443935+Dan-Heath@users.noreply.github.com>

---------

Co-authored-by: Dan Heath <76443935+Dan-Heath@users.noreply.github.com>

website/content/docs/api-clients/client-agent.mdx

@@ -534,12 +534,24 @@ To configure the DNS server(s) to use, use the `override_upstream_dns_servers` c
 # The DNS servers must be specified as an IP, or an IP:Port.
 # If no port is provided, port 53 is assumed.
 # The order of the entries specifies the priority.
+# We recommended providing both the VPN DNS servers
+# and the default DNS servers, so that DNS requests can
+# be resolved even when the VPN is not active.
 override_upstream_dns_servers = [
-   "8.8.8.8",
-   "8.8.4.4:53",
+   "10.0.0.1", # Example primary VPN DNS server
+   "10.0.0.2", # Example secondary VPN DNS server
+   "8.8.8.8",  # Fallback default DNS server
+   "8.8.4.4:53", # Fallback default DNS server with a custom port
 ]
 ```
 
+<Note>
+
+The `override_upstream_dns_servers` is used for all non-Boundary DNS requests.
+If you only provide the VPN DNS servers, the Client Agent will not be able to resolve any DNS requests when the VPN is not active.
+
+</Note>
+
 #### Primary network interfaces
 
 By default, the Client Agent creates IPs on the primary network interface to serve its DNS server.