fix #7560: user management: privileges: document Sys.AccessNetwork

The Sys.AccessNetwork privilege got added with pve-access-control
commit 36c1814 ("add Sys.AccessNetwork privilege") and is part of the
built-in Administrator role, but it is missing in the list of
privileges.

Document it with examples from the API calls that currently check for
it, which are downloading ISO images, templates and other images from a
URL to a storage and querying the metadata of such a URL, querying the
tags of and pulling OCI images, and creating or updating notification
targets.

Signed-off-by: Michal Fox <me@dualfroz.com>

pveum.adoc

@@ -889,6 +889,9 @@ Node / System related privileges::
 * `Realm.Allocate`: create/modify/remove authentication realms
 * `SDN.Allocate`: manage SDN configuration
 * `SDN.Audit`: view SDN configuration
+* `Sys.AccessNetwork`: access the network from the node, for example to
+  download ISO images or templates from a URL, to pull OCI images or to set up
+  notification targets
 * `Sys.Audit`: view node status/config, Corosync cluster config, and HA config
 * `Sys.Console`: console access to node
 * `Sys.Incoming`: allow incoming data streams from other clusters (experimental)