Document `RactorLocalSingleton`

I was made aware of the RactorLocalSingleton class today. When I attempted to look it up in the Ruby docs, I was greeted with an empty page on both 3.4 and master:

• https://docs.ruby-lang.org/en/3.4/RactorLocalSingleton.html
• https://docs.ruby-lang.org/en/master/RactorLocalSingleton.html

It's not clear what the advice about when or whether to use this module is. I see that there is some discussion around the time the feature was implemented:

• https://bugs.ruby-lang.org/issues/18127
• https://github.com/ruby/singleton/pull/4

It would be nice if someone who designed this feature could clarify what the intended usage and possible limitations of this module are, so that the documentation gets published to the Ruby Documentation site.

まず Ruby 3.4 と master の空の RactorLocalSingleton ページを確認し、その後、機能の背景を把握するために bugs.ruby-lang.org/issues/18127 と ruby/singleton#4 を読んでください。関連する設計上の背景を踏まえて想定される使用方法と制限事項を明確にし、Ruby Documentation サイトに有用なドキュメントが公開された時点で作業完了と考えてください。