Document default revision installation changes (#10450)

* Document default revision changes

* Remove extra space

Co-authored-by: craigbox <craigbox@google.com>

* Remove additional with

Co-authored-by: craigbox <craigbox@google.com>

Co-authored-by: craigbox <craigbox@google.com>

content/en/boilerplates/revision-tags-default-intro.md

@@ -1,7 +1,11 @@
 ---
 ---
-The revision pointed to by the tag `default` is considered the ***default revision*** and has additional semantic meaning.
+The revision pointed to by the tag `default` is considered the ***default revision*** and has additional semantic meaning. The default
+revision performs the following functions:
 
-The `default` revision will inject sidecars for the `istio-injection=enabled` namespace selector and `sidecar.istio.io/inject=true` object
-selector in addition to the `istio.io/rev=default` selectors. This makes it possible to migrate from using non-revisioned Istio to using
-a revision entirely without relabeling namespaces. To make a revision `1-10-0` the default, run:
+- Injects sidecars for the `istio-injection=enabled` namespace selector, the `sidecar.istio.io/inject=true` object
+  selector, and the `istio.io/rev=default` selectors
+- Validates Istio resources
+- Steals the leader lock from non-default revisions and performs singleton mesh responsibilities (such as updating resource statuses)
+
+To make a revision `1-10-0` the default, run:

content/en/docs/setup/install/helm/index.md

@@ -26,6 +26,11 @@ Follow this guide to install and configure an Istio mesh using
 1. Install the Istio base chart which contains cluster-wide resources used by
    the Istio control plane:
 
+    {{< warning >}}
+    When performing a revisioned installation, the base chart requires the `--defaultRevision` value to be set for resource
+    validation to function. More information on the `--defaultRevision` option can be found in the Helm upgrade documentation.
+    {{< /warning >}}
+
     {{< text syntax=bash snip_id=install_base >}}
     $ helm install istio-base istio/base -n istio-system
     {{< /text >}}

content/en/docs/setup/upgrade/helm/index.md

@@ -76,10 +76,10 @@ primary and canary installations.
     $ helm delete istiod -n istio-system
     {{< /text >}}
 
-1. Upgrade the Istio base chart:
+1. Upgrade the Istio base chart, making the new revision the default.
 
     {{< text bash >}}
-    $ helm upgrade istio-base istio/base -n istio-system --skip-crds
+    $ helm upgrade istio-base istio/base --defaultRevision canary -n istio-system --skip-crds
     {{< /text >}}
 
 ### Stable revision labels (experimental)