Exempt MemberNotNullWhen from genapi ref assembly requirements (dotne…

…t#70113)

As with MemberNotNull, MemberNotNullWhen is an implementation detail when it refers to a private member (even when attributed on a public API).  The tooling shouldn't require it in reference assemblies, as in such situations it a) isn't meaningful to a consumer and b) leaks implementation details.

eng/DefaultGenApiDocIds.txt

@@ -5,6 +5,7 @@ T:System.Configuration.ConfigurationPropertyAttribute
 T:System.Diagnostics.CodeAnalysis.DynamicDependencyAttribute
 T:System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverageAttribute
 T:System.Diagnostics.CodeAnalysis.MemberNotNullAttribute
+T:System.Diagnostics.CodeAnalysis.MemberNotNullWhenAttribute
 T:System.Diagnostics.CodeAnalysis.SuppressMessageAttribute
 T:System.Diagnostics.CodeAnalysis.UnconditionalSuppressMessageAttribute
 T:System.Diagnostics.DebuggerBrowsableAttribute