Add missing anchor link (#22427)

Co-authored-by: Rachael Shaw <r@rachael.wtf>

docs/REST API/rest-api.md

@@ -8644,6 +8644,7 @@ Deletes the session specified by ID. When the user associated with the session n
 - [Get software version](#get-software-version)
 - [Get operating system version](#get-operating-system-version)
 - [Add package](#add-package)
+- [Modify package](#modify-package)
 - [List App Store apps](#list-app-store-apps)
 - [Add App Store app](#add-app-store-app)
 - [Add Fleet library app](#add-fleet-library-app)
@@ -9230,6 +9231,8 @@ Content-Type: application/octet-stream
 
 ### Modify package
 
+> **Experimental feature**. This feature is undergoing rapid improvement, which may result in breaking changes to the API or configuration surface. It is not recommended for use in automated workflows.
+
 _Available in Fleet Premium._
 
 Update a package to install on macOS, Windows, or Linux (Ubuntu) hosts.