Chat: Update message input placeholder to mention slash commands (#2142)

Now commands are staying in for release, time to update the input
placeholder:

| Before | After |
| - | - |
| <img width="436" alt="Screenshot 2023-12-06 at 8 23 21 pm"
src="https://github.com/sourcegraph/cody/assets/153/90386905-c3bd-4bdc-886e-713743441775">
| <img width="436" alt="Screenshot 2023-12-06 at 8 22 02 pm"
src="https://github.com/sourcegraph/cody/assets/153/08f036db-00c1-4121-bf9c-47400c27e77e">
|

## Test plan

- Opened a chat
- Inspected placeholder

vscode/CHANGELOG.md

@@ -38,6 +38,7 @@ This is a log of all notable changes to Cody for VS Code. [Unreleased] changes a
 - Commands: Expose commands in the VS Code command palette and clean up the context menu. [pull/1209](https://github.com/sourcegraph/cody/pull/2109)
 - Search: Style and UX improvements to the search panel. [pull/2138](https://github.com/sourcegraph/cody/pull/2138)
 - Chat: Reduce size of chats list blank copy. [pull/2137](https://github.com/sourcegraph/cody/pull/2137)
+- Chat: Update message input placeholder to mention slash commands. [pull/2142](https://github.com/sourcegraph/cody/pull/2142)
 
 ## [0.16.3]
 

vscode/webviews/Chat.tsx

@@ -269,7 +269,7 @@ const TextArea: React.FunctionComponent<ChatUITextAreaProps> = ({
     chatModels,
 }) => {
     const inputRef = useRef<HTMLTextAreaElement>(null)
-    const placeholder = 'Message (type @ to include files)'
+    const placeholder = 'Message (@ to include code, / for commands)'
 
     useEffect(() => {
         if (autoFocus) {