The big red box at the top even tells you that, and gives you a link to the up to date Jetty documentation.
Windows File Locking is a standard feature of windows. Files that are open are locked and prevented from being tampered with.
You cannot rely on this feature of Windows for what you want. As Jetty cannot have all files on your webapp open at the same time, that would hit open file limits on your OS rather quickly.
The temporary directory of a webapp is standard servlet spec behavior and is where all temporary files reside, that directory can never be read-only, otherwise your webapp wouldn't function as you expect.
You should probably just rely on standard operating system protections, have Jetty run on it's own user, with it's own temporary directory, setup to not allow other users on the operating system to change/modify the content of those directories.
This is not something that can be controlled by Jetty itself, this is something you have to setup and configure at the OS level, the FileSystem level, and perhaps even configure your WebApp to use those user specific directories.