Clarify how mapping constructor infers a type

Fixes #54.

lang/spec.html

@@ -2631,9 +2631,18 @@ <h3>Mapping constructor</h3>
 that has a default value.
 </p>
 <p>
-If there is no contextually expected type, the inherent type is inferred from
-the field names and the static types of the expressions occurring in the mapping
-constructor.
+If there is no contextually expected type, the type descriptor for the inherent
+type will be inferred from the field names and the static types of the
+expressions occurring in the mapping constructor. The inferred type will have
+field-descriptor for each literal-field-name occurring in the constructor; each
+field-descriptor will be required, and the static type will the broad type of
+the value-expr for the field; the type will be open to
+<code>anydata|error</code>, just as if it had been specified using <code>record
+{ <var>field-descriptors</var> }</code>.
+</p>
+<p>
+A value-expr will not have a contextually expected type if its
+mapping-constructor-expr does not have a contextually expected type.
 </p>
 <h3>[Preview] Table constructor</h3>