[SYCL][Doc] Add device global extension spec (#4233)

Signed-off-by: Michael Kinsner <[email protected]>
Co-authored-by: John Pennycook <[email protected]>
Co-authored-by: Ronan Keryell <[email protected]>
Co-authored-by: Greg Lueck <[email protected]>
Co-authored-by: Alexey Bader <[email protected]>

Author: 5 people