In the last two or three years, I've written "What's Here" sections for about 20 Ruby core and standard library classes and modules.

Here's the one for class Array: https://docs.ruby-lang.org/en/master/Array.html#class-Array-label-What-27s+Here.

It's not a tutorial; it's an overview. It's meant to help readers become acquainted with the methods in the class.