[SYCL][Doc] Update ExtendedAtomics documentation

Several major changes to the extension:
- No longer replaces cl::sycl::atomic
- Closer alignment with C++20 atomic_ref
- Supports additional memory orders

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

Author: Pennycook

sycl/doc/extensions/ExtendedAtomics/README.md

@@ -1,3 +1,3 @@
 # SYCL_INTEL_extended_atomics
 
-Replaces the `cl::sycl::atomic` class with the `cl::sycl::intel::atomic` class, which exposes additional functionality aligned with the `std::atomic` class from C++11 and C++20.
+Introduces the `cl::sycl::intel::atomic_ref` class, which exposes additional functionality aligned with the `std::atomic_ref` class from C++20.