Update Metadata section in params-collections.md (#7850)

proposals/params-collections.md

@@ -498,7 +498,21 @@ the construction of parameter collections when APIs are invoked in their expande
 
 ### Metadata
 
-In metadata `params` parameters are marked with `System.ParamArrayAttribute` as `params` arrays are marked today.
+In metadata we could mark non-array `params` parameters with `System.ParamArrayAttribute`, as `params` arrays are marked today.
+However, it looks like we will be much safer to use a different attribute for non-array `params` parameters.
+For example, the current VB compiler will not be able to consume them decorated with `ParamArrayAttribute` neither in normal, nor in expanded form. Therefore, an addition of 'params' modifier is likely to break VB consumers, and very likely consumers from other languages or tools.
+
+Given that, non-array `params` parameters are marked with a new `System.Runtime.CompilerServices.ParamCollectionAttribute`.
+```
+namespace System.Runtime.CompilerServices
+{
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = true, AllowMultiple = false)]
+    public sealed class ParamCollectionAttribute : Attribute
+    {
+        public ParamCollectionAttribute() { }
+    }
+}
+```
 
 ## Open questions