[SYCL][Doc] Attempt to clarify group barrier fence

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

Author: Pennycook

sycl/doc/extensions/GroupAlgorithms/SYCL_INTEL_group_algorithms.asciidoc

@@ -260,14 +260,16 @@ The return types of the collective functions in {cpp}17 are not deduced from the
 
 ==== Synchronization
 
+The behavior of memory fences in this section is aligned with the single happens-before relationship defined by the +SYCL_INTEL_extended_atomics+ extension.
+
 |===
 |Function|Description
 
 |+template <typename Group> void barrier(Group g);+
-|Synchronize all work-items in the group, and ensure that all memory accesses prior to the barrier are visible to all work-items in the group after the barrier.  The scope of the group memory fences implied by this barrier is the narrowest scope including all work-items in the group.
+|Synchronize all work-items in the group, and ensure that all memory accesses to any address space prior to the barrier are visible to all work-items in the group after the barrier.  The scope of the group memory fences implied by this barrier is the narrowest scope including all work-items in the group.
 
 |+template <typename Group> void barrier(Group g, memory_scope scope);+
-|Synchronize all work-items in the group, and ensure that all memory accesses prior to the barrier are visible to all work-items in the group after the barrier.  The scope of the group memory fences implied by this barrier is specified by _scope_.
+|Synchronize all work-items in the group, and ensure that all memory accesses to any address space prior to the barrier are visible to all work-items specified by _scope_ after the barrier.  The scope of the group memory fences implied by this barrier is controlled by _scope_ and must be broader than the narrowest scope including all work-items in the group.
 
 |===