modules/understanding-upgrade-channels: Recommend clearing channel

I would prefer to configure this by clearing the 'upstream' setting,
which seems more intuitive for "there is no upstream" to me.  But
sadly, it seems that the CVO has been falling back to a default URI
when the ClusterVersion upstream is empty since way back [1,2], and
that this behavior is enshrined in the API [3].  Although the channel
docs also talk about defaults [4], the only channel defaulting in the
CVO is when creating a ClusterVersion object after the in-cluster copy
was (accidentally?)  deleted [5].  So maybe we could talk folks into
adjusting the CVO logic to return NoUpstream in the empty-upstream
case, but at the moment, clearing the channel is the best approach for
"the CVO keeps complaining that it can't hit the upstream, and I want
ot quiet it down [6]".

The 'oc adm upgrade channel' command just landed for 4.9 in [7].

[1]: https://github.com/openshift/cluster-version-operator/blame/2c4931dc283c551938be1a00fee290de0b79d99c/pkg/cvo/availableupdates.go#L27-L31
[2]: openshift/cluster-version-operator@ab4d84a#diff-78f2af341fa49292dd6930f378018867R24
[3]: https://github.com/openshift/api/blame/0422dc17083e9e8df18d029f3f34322e96e9c326/config/v1/types_cluster_version.go#L56-L57
[4]: https://github.com/openshift/api/blame/0422dc17083e9e8df18d029f3f34322e96e9c326/config/v1/types_cluster_version.go#L62-L63
[5]: https://github.com/openshift/cluster-version-operator/blob/2c4931dc283c551938be1a00fee290de0b79d99c/pkg/cvo/cvo.go#L602
[6]: https://bugzilla.redhat.com/show_bug.cgi?id=1827378
[7]: openshift/oc#576

Author: wking

modules/understanding-upgrade-channels.adoc

@@ -21,6 +21,8 @@ ifndef::openshift-origin[]
 * `stable-{product-version}`
 * `eus-4.6` (only available when running 4.6)
 
+If you do not want the Cluster Version Operator to fetch available updates from the upgrade recommendation service, you can use the `oc adm upgrade channel` command in the OpenShift CLI to configure an empty channel. This configuration can be helpful if, for example, a cluster has restricted network access and there is no local, reachable upgrade recommendation service.
+
 endif::openshift-origin[]
 ifdef::openshift-origin[]
 {product-title} {product-version} offers the following upgrade channel:

updating/updating-cluster-between-minor.adoc

@@ -9,7 +9,7 @@ You can update, or upgrade, an {product-title} cluster between minor versions.
 
 [NOTE]
 ====
-Because of the difficulty of changing update channels by using `oc`, use the web console to change the update channel. It is recommended to complete the update process within the web console. You can follow the steps in xref:../updating/updating-cluster-cli.adoc#updating-cluster-cli[Updating a cluster within a minor version by using the CLI] to complete the update after you change to a {product-version} channel.
+Use the web console or `oc adm upgrade channel _<channel>_` to change the update channel. You can follow the steps in xref:../updating/updating-cluster-cli.adoc#updating-cluster-cli[Updating a cluster within a minor version by using the CLI] to complete the update after you change to a {product-version} channel.
 ====
 
 == Prerequisites