section/collectives: Clarify UB for data races with collectives

Closes openshmem-org#74

content/collective_intro.tex

@@ -18,6 +18,12 @@
   required, the default context.
 \end{enumerate}
 
+Concurrent accesses to symmetric memory by an \openshmem collective
+routine and any other means of access---where at least one updates the
+symmetric memory---results in undefined behavior.
+Since \acp{PE} can enter and exit collectives at different times,
+accessing such memory remotely may require additional synchronization.
+
 \subsubsection*{Team-based collectives}
 
 The team-based collective routines are performed with respect to a valid