Add missing documentation

Author: eiriktsarpalis

xml/System.Text.Json.Serialization.Metadata/JsonPolymorphismOptions.xml

@@ -31,7 +31,7 @@
       </AssemblyInfo>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Creates an empty <see cref="T:System.Text.Json.Serialization.Metadata.JsonPolymorphismOptions" /> instance.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>