Missing doc for UnmanagedCallersOnlyAttribute.CallConvs. (#4764)

* Missing doc for UnmanagedCallersOnlyAttribute.CallConvs.

* Update xml/System.Runtime.InteropServices/UnmanagedCallersOnlyAttribute.xml

Co-authored-by: Carlos Sanchez <[email protected]>

Co-authored-by: Carlos Sanchez <[email protected]>

Author: AaronRobinsonMSFT

xml/System.Runtime.InteropServices/UnmanagedCallersOnlyAttribute.xml

@@ -68,8 +68,16 @@ Methods marked with this attribute have the following restrictions:
         <ReturnType>System.Type[]</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>To be added.</summary>
-        <remarks>To be added.</remarks>
+        <summary>Optional. If omitted, the runtime will use the default platform calling convention.</summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+Supplied types must be from the official <xref:System.Runtime.CompilerServices> namespace and be of the form `CallConvXXX`.
+        
+          ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="EntryPoint">