Update fleet-agent-proxy-package-registry.asciidoc (#1722) (#1724)

* Update fleet-agent-proxy-package-registry.asciidoc

* Update docs/en/ingest-management/fleet-agent-proxy-package-registry.asciidoc

---------

Co-authored-by: David Kilfoyle <41695641+kilfoyle@users.noreply.github.com>
(cherry picked from commit 73cea8c)

Co-authored-by: Luca Belluccini <luca.belluccini@elastic.co>

docs/en/ingest-management/fleet-agent-proxy-package-registry.asciidoc

@@ -17,6 +17,10 @@ example:
 xpack.fleet.registryProxyUrl: your-nat-gateway.corp.net
 ----
 
+If your HTTP proxy requires authentication, you can include the 
+credentials in the URI, such as `https://username:password@your-nat-gateway.corp.net`, 
+only when using HTTPS.
+
 == What information is sent to the {package-registry}?
 
 In production environments, {kib}, through the {fleet} plugin, is the only service interacting with the {package-registry}. Communication happens when interacting with the Integrations UI, and when upgrading {kib}. The shared information is about discovery of Elastic packages and their available versions. In general, the only deployment-specific data that is shared is the {kib} version.