Update example ADS bootstrap config to specify TCP Keep-Alive options (#6231)

Update ADS protocol docs to specify a Keep-Alive so others consider adding it in. If the Keep-Alive parameters are not specified, it's very easy to end up in a TCP Half Open state during a network glitch/force disconnect where the Envoy thinks it's connected to the ADS server when it's not.

Risk Level: Low
Testing:
Docs Changes: N/A
Release Notes: N/A

Somewhat addresses: #5173

Signed-off-by: Suhail Patel <me@suhailpatel.com>

Mirrored from https://github.com/envoyproxy/envoy @ 0286b82280d33d39b5fec7dbc6be34d2fc943c7d

XDS_PROTOCOL.md

@@ -297,6 +297,11 @@ static_resources:
         port_value: <ADS management server port>
     lb_policy: ROUND_ROBIN
     http2_protocol_options: {}
+    upstream_connection_options:
+      # configure a TCP keep-alive to detect and reconnect to the admin
+      # server in the event of a TCP socket disconnection
+      tcp_keepalive:
+        ...
 admin:
   ...