Rubyists don't call these things annotations. One of the things I like doing is to find common techniques that cross languages, for me this is a common technique and 'annotation' seems like a good generic word for it. I don't know if Rubyists will agree.

Languages may provide annotations in ways that don't reflect the syntax of the language

When writing about programming, I prefer to use 'annotation' as the general term. Although .NET was first, the word 'attribute' is just too widely used for different things.

An annotation on a program element (commonly a class, method, or field) is a piece of meta-data added to that program element which can be used to embellish that element with extra code.

Now you can easily spot them! The rule is: if you see a Result it means that this function can throw an exception. And you even know its type in advance.