Update copy to mention that orbit has access to all installers (#21342)

Author: dantecatalfamo

docs/REST API/rest-api.md

@@ -9178,6 +9178,8 @@ _Available in Fleet Premium._
 
 Install software (package or App Store app) on a macOS, iOS, iPadOS, Windows, or Linux (Ubuntu) host. Software title must have a `software_package` or `app_store_app` added to be installed.
 
+> Note: Fleet's agent (fleetd) only installs software it has been asked to install, but technically has access to all installer executables.
+
 `POST /api/v1/fleet/hosts/:id/software/install/:software_title_id`
 
 #### Parameters