Add note about work-item scope atomics

In OpenCL, these atomics are only required to support a very specific use-case
involving images, and are forbidden in all other contexts.

In SYCL, we would like a work-item to be viewed as a degenerate case of a group
containing a single work-item. Work-item scope atomics should thus be
permitted, and their effect should be equivalent to non-atomic operations.

Author: Pennycook

adoc/chapters/architecture.adoc

@@ -1089,6 +1089,13 @@ values:
     the memory allocation containing the referenced object, as defined by the
     capabilities of <<buffer,buffers>> and <<usm>>.
 
+{note}An atomic operation with work-item scope is effectively the same as a
+non-atomic operation.
+[code]#sycl::memory_scope::work_item# is primarily intended to simplify generic
+programming and to provide a meaningful way to describe the behavior of
+<<group,groups>> containing a single work-item.
+{endnote}
+
 The memory scopes are listed above from narrowest
 ([code]#memory_scope::work_item#) to widest ([code]#memory_scope::system#).