Improve ?Sized diagnostics

[mgattozzi]

Context: https://twitter.com/mgattozzi/status/1229477994961494021

A way to improve the diagnostics when suggesting to relax trait bounds with ?Sized is to show how one would use it on a struct/fn with their code rather than just suggesting it, making it easier for newer people, or those first encountering the issue to more clearly understand how this works.

cc/ @estebank

[LeSeulArtichaut]

What about having a separate error code, so that --explain can give details and examples about DSTs?

[estebank]

I think that we get 80% there by using span_suggestion_verbose instead of span_suggestion and an extra 10% by adding a note explaining a bit of this. The last 10% would be a specific error code, but we have shied away from replacing E0277s in the past.