fix: nodoc deprecation fix (#949)

* Fix `nodoc` deprecation warning.

Running "dart doc ." in `packages/win32` previously generated a warning that "  warning: deprecated dartdoc usage: The '--nodoc' option is deprecated, and will soon be removed.
    from package-win32: file:///C:/Users/weasd/Downloads/forks/win32/packages/win32". This is fixed by this commit, which replaces the deprecated `nodoc` option in `dartdoc_options.yaml` with a `@nodoc` annotation in `constants_nodoc.dart`.

* Clarify comment

packages/win32/dartdoc_options.yaml

@@ -107,7 +107,6 @@ dartdoc:
       'examples',
     ]
   showUndocumentedCategories: true
-  nodoc: ['lib/src/constants_nodoc.dart']
   ignore:
     - broken-link
     - missing-from-search-index

packages/win32/lib/src/constants_nodoc.dart

@@ -3,10 +3,6 @@
 // *** NOTE: This file contains constants that we have chosen not to document,
 // mostly because their function is obvious, or because they are documented in
 // the public Microsoft Windows documentation online.
-//
-// This file (only) is excluded from dartdoc in `dartdoc_options.yaml`. By
-// keeping non-documented constants in a separate file, we save ourselves the
-// need to add the /// @nodoc modifier to every single constant in this file.
 
 // Contributors are invited and encouraged to submit comments for these
 // constants from the open source Microsoft documentation, such as
@@ -16,6 +12,10 @@
 // ignore_for_file: camel_case_types, constant_identifier_names
 // ignore_for_file: non_constant_identifier_names
 
+// Do not document anything in this library.
+/// @nodoc
+library;
+
 import 'dart:ffi';
 
 import 'package:ffi/ffi.dart';