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.

首先查看 Ruby 3.4 和 master 中空的 RactorLocalSingleton 页面，然后阅读 bugs.ruby-lang.org/issues/18127 和 ruby/singleton#4，了解该功能的背景。结合相关设计背景，明确预期用法和限制，并在 Ruby Documentation 网站发布有用的文档后视为工作完成。