Added generic example of std::ops::Add in doc comments
We discussed on IRC how the std::ops examples were potentially missing examples using generics. This PR adds an example to std::ops::Add that shows the use of a generic type T. I'm not sure this is ready for merge as I think the two examples now make the documentation a bit verbose, but I think it's a good starting point. I'd love to hear others thoughts on this. This is in relation to the last item in issue #29365.