[SYCL][Doc] Deprecate extended atomics extension (#5983)

The version of atomic_ref from this extension has been deprecated in
favor of sycl::atomic_ref from SYCL 2020.

atomic_accessor has also been deprecated, in favor of creating a
sycl::atomic_ref from a read-write accessor.

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook