Add vtctldclient info to the 18.0 summary (#14259)

Signed-off-by: Matt Lord <mattalord@gmail.com>

changelog/18.0/18.0.0/summary.md

@@ -14,6 +14,7 @@
   - **[VTAdmin](#vtadmin)**
     - [Updated to node v18.16.0](#update-node)
   - **[Deprecations and Deletions](#deprecations-and-deletions)**
+    - [Legacy Client Binaries](#legacy-client-binaries)
     - [Deprecated Flags](#deprecated-flags)
     - [Deprecated Stats](#deprecated-stats)
     - [Deleted Flags](#deleted-flags)
@@ -109,6 +110,28 @@ on https://nodejs.org/en/blog/release/v18.16.0.
 
 ### <a id="deprecations-and-deletions"/>Deprecations and Deletions
 
+#### <a id="legacy-client-binaries"/>Legacy Client Binaries
+
+`vtctldclient` is our new modern *Vitess controller daemon* (`vtctld`) *client* – which you will use to perform commands
+and take actions in your Vitess clusters. It is [replacing the legacy `vtctl`/`vtctlclient` binaries](https://vitess.io/docs/18.0/reference/vtctldclient-transition/overview/).
+Some of the benefits are:
+
+- [Dedicated RPCs for each command](https://github.com/vitessio/vitess/blob/release-18.0/proto/vtctlservice.proto#L32-L353)
+that are used between `vtctldclient` and `vtctld` – this offers clean separation of commands and makes it easier to
+develop new features without impacting other commands. This also presents an [API that other clients (both Vitess and
+3rd party) can use to interface with Vitess](https://vitess.io/blog/2023-04-17-vtctldserver-api/).
+- Use of modern frameworks: [`pFlag`](https://github.com/spf13/pflag#readme), [`Cobra`](https://cobra.dev), and [`Viper`](https://github.com/spf13/viper#readme).
+This makes development easier while also offering a better UX. For example, this offers a way to use
+[configuration files](https://vitess.io/docs/18.0/reference/viper/config-files/) with support for
+[dynamic configuration](https://vitess.io/docs/18.0/reference/viper/dynamic-values/) ([see also](https://github.com/vitessio/vitess/blob/release-18.0/doc/viper/viper.md)).
+- The [reference documentation](https://vitess.io/docs/reference/programs/vtctldclient/) is now built through code. This
+removes a burden from developers while helping users by ensuring the docs are always correct and up-to-date.
+
+In Vitess 18.0 we have completed migrating all client commands to `vtctldclient` – the last ones being the [OnlineDDL](https://github.com/vitessio/vitess/issues/13712)
+and [VReplication](https://github.com/vitessio/vitess/issues/12152) commands. With this work now completed, the
+legacy `vtctl`/`vtctlclient` binaries are now fully deprecated and we plan to remove them in Vitess 19.0. You should
+[begin your transition](https://vitess.io/docs/18.0/reference/vtctldclient-transition/) before upgrading to 18.0.
+
 #### <a id="deprecated-flags"/>Deprecated Command Line Flags
 
 Throttler related `vttablet` flags: