Clarify behavior of atomic_ref DefaultScope

An implementation of atomic_ref<T> that was not lock-free needs to know which
work-items may access the lock in order to decide where to allocate the lock.

This additionally serves as a clarification of the behavior of work-item scope;
using an atomic_ref with work-item scope at the same time as an atomic_ref
with broader scope is invalid.

Author: Pennycook

adoc/chapters/programming_interface.adoc

@@ -20340,6 +20340,10 @@ scope for the atomic operations.
 Most member functions also provide an optional parameter that allows the
 application to override this default.
 
+All accesses to an object referenced by an [code]#sycl::atomic_ref# must
+exclusively occur through instances of an [code]#sycl::atomic_ref# with the same
+[code]#DefaultScope#.
+
 The [code]#sycl::atomic_ref# class also has a template parameter
 [code]#AddressSpace#, which allows the application to make an assertion about
 the address space of the object of type [code]#T# that it references.