[intro.races] Clarify 'synchronization operation'.

Fixes CWG 2297.

Author: jensmaurer

source/basic.tex

@@ -4519,9 +4519,12 @@
 memory location.
 
 \pnum
-The library defines a number of atomic operations\iref{atomics} and
-operations on mutexes\iref{thread} that are specially identified as
-synchronization operations. These operations play a special role in making
+The library identifies certain operations as
+\defnx{synchronization operations}{synchronization operation}, as specified
+in the \sync element
+(\ref{support.signal}, \ref{util.smartptr.shared.atomic}, \ref{atomics.order},
+\ref{atomics.fences}, \ref{thread}).
+These operations play a special role in making
 assignments in one thread visible to another. A synchronization operation on one
 or more memory locations is either a consume operation, an acquire operation, a
 release operation, or both an acquire and release operation. A synchronization