Add upgrade clients documentation. (#3067)

Co-authored-by: Sean Chen <[email protected]>

.changelog/unreleased/bug-fixes/ibc-relayer-cli/3066-upgrade-clients-docs.md

@@ -0,0 +1,2 @@
+- Document upgrade clients command
+  ([#3066](https://github.com/informalsystems/hermes/issues/3066))

guide/src/documentation/commands/upgrade/index.md

@@ -1,13 +1,21 @@
-# Client Upgrade
+# Upgrading Clients
+If IBC clients need to be upgraded after their reference chains went through an upgrade, the following CLIs may be used.
 
-## Client Upgrade Command
-
-Use the `upgrade client` command to upgrade a client after a chain upgrade.
+## Upgrade Client Command
+Use the `upgrade client` command to upgrade a specific IBC client after a chain upgrade.
 
 ```shell
 {{#include ../../../templates/help_templates/upgrade/client.md}}
 ```
 
+## Upgrade Clients Command
+
+Use the `upgrade clients` command to upgrade all IBC clients that target a specific (upgraded) chain.
+
+```shell
+{{#include ../../../templates/help_templates/upgrade/clients.md}}
+```
+
 __Example__
 
 Here is [an example](./test.md) of a chain upgrade proposal submission and client upgrade.