Document `RactorLocalSingleton`

[jez]

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
• Create RactorLocalSingleton #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.