Update using-directive.md

using static doesn't require a fully qualified type if it is in the scope of a namespace.
see:
https://github.com/dotnet/csharpstandard/blob/standard-v6/standard/namespaces.md#1354-using-static-directives
here the grammar is:
> using_static_directive
>    : 'using' 'static' type_name ';'
>    ;

Author: AdmiralSnyder

docs/csharp/language-reference/keywords/using-directive.md

@@ -82,11 +82,14 @@ Analyzers issue diagnostics if you duplicate `global` using directives in differ
 The `using static` directive names a type whose static members and nested types you can access without specifying a type name. Its syntax is:
 
 ```csharp
+// not within a namespace
 using static <fully-qualified-type-name>;
 ```
 
 The `<fully-qualified-type-name>` is the name of the type whose static members and nested types can be referenced without specifying a type name. If you don't provide a fully qualified type name (the full namespace name along with the type name), C# generates compiler error [CS0246](../compiler-messages/assembly-references.md): "The type or namespace name 'type/namespace' couldn't be found (are you missing a using directive or an assembly reference?)".
 
+If the `using static` directive is applied within the context of a namespace (either file-scoped or nested in a `namespace` block, it is not necessary to fully qualify the type.
+
 The `using static` directive applies to any type that has static members (or nested types), even if it also has instance members. However, instance members can only be invoked through the type instance.
 
 You can access static members of a type without having to qualify the access with the type name: