Adjust Set documentation

[zverok]

• Change #inspect result throughout the docs according to 4.0 rendering
• Add a section about nuances of inheritance and compatibility
• Fix TOC (for some reason referred to Array's docs 🤷)
• Adjusted documentation for #to_set, explaining the deprecation of a form with arguments;
• A few minor tweaks of rendering/formatting;
• exclude set/subclass_compatible.rb from documentation. The module itself never rendered, as it is private, but the file header was included in the main Set docs, which looked weird. I didn't find away to remove it with :stopdoc: comments, for some reason, so excluded it via RDoc options file.

[jeremyevans]

Thank you for fixing these issues!

[jeremyevans]

My preference would be to change the call-seq to show no arguments, and only document the behavior with no arguments. We are removing argument handling right after the release of 4.0.

[zverok]

I believe it is better to keep the documentation aligned with the real code. Otherwise, if the user "discovers" there are other arguments, they wouldn't know whether it is something "deprecated and not recommended to use", or something "very new, nobody had time to document yet" (this happens sometimes).

And in general, it might make a bad impression of the language if the real call-sequence (however they "discovered" it) doesn't correspond to the documented one.

I believe that for now, the most clear way is to document documents as they are, but clearly state that using them is deprecated.