Document the ref keyword

Partial fix for #34601.

This documents the `ref` keyword with two examples, one failing to compile because the `ref` keyword is missing, and the same example fixed with the keyword inserted in the correct place.

It also explains (very *very* rapidly) the differences between `&` and `ref`.

I put a link to the best place I could find in the Reference but there may be something better that I didn't find.