Clarify how deref coercions interact with traits

[Spartan2909]

The book currently doesn't state that deref coercions don't always happen when matching traits, which has led to some confusion (see rust-lang/rust#109829). The nomicon does state this, but that is aimed at a different audience to the book.

[chriskrycho]

I think this is valuable information, but I don’t think it makes sense to have it here. We don’t discuss fully-qualified syntax until Chapter 20, and for good reason: it’s a pretty advanced topic. I just reread that section, and unfortunately, I don’t think this fits there particularly well, either, though—mostly because it’s just one of those “this is an edge case that does matter but isn’t really part of ‘normal’/day-to-day Rust”. (I think I probably write a Trait::method() once for every hundred times I write it in the method format, if that!)

I agree this should be documented; you might file an issue on the Rust Reference and see if and how it makes sense to add to that, instead?