Semantics of table key-specifier in query-expr

Part of #47.

lang/spec.html

@@ -5006,7 +5006,10 @@ <h3>Query expression</h3>
 every emitted value must be of type xml</li>
 </ul>
 <p>
-The applicable contextually expected type determines which basic type is
+If the query-expr starts with <code>table</code>, then the query-expr will
+construct a table; the key-specifier specifies the key sequence of the
+constructed table in the same way as with a table-constructor-expr. Otherwise,
+the applicable contextually expected type determines which basic type is
 constructed. If there is no contextually expected type, then it is determined
 from the static type of the expression in the initial <code>from</code> clause;
 it is a compile-time error if this static type is not a subtype of one of the