Semantics of readonly in mapping constructor

Part of  #464

lang/spec.html

@@ -3628,8 +3628,13 @@ <h3>Mapping constructor</h3>
 order in which the computed-name-fields occur in the mapping-constructor-expr.
 </p>
 <p>
-An mapping-constructor-expr occurring within a const-expr will construct a mapping value
-that has its read-only bit on.
+If the applicable contextually expected type is a subtype of readonly, then the
+mapping will be constructed with its read-only bit on. If the inherent type
+makes a specific field readonly, then that field will be constructed with its
+read-only bit on. A specific-field that starts with <code>readonly</code> will
+also be constructed with its read-only bit on. An mapping-constructor-expr
+occurring within a const-expr will construct a mapping value that has its
+read-only bit on.
 </p>
 
 </section>