Explain transactiion nesting

Part of #267.

lang/spec.html

@@ -8753,6 +8753,13 @@ <h4>Transaction statement</h4>
 any statements that could potentially be executed after the execution of a
 commit-action or rollback-stmt.
 </p>
+<p>
+It is an error for a transactional-stmt to appear in a transactional scope.
+However, it is not an error if the current strand is in transaction mode when a
+transaction-stmt is executed: in this case, the Begin() operation performed by
+the transaction-stmt will, as usual, start a new transaction, which will be
+completely independent of any existing transactions.
+</p>
 </section>