Fill in prose to describe the `async_fn_in_trait` lint

We're stabilizing `async fn` in trait (AFIT), but we have some
reservations about how people might use this in the definitions of
publicly-visible traits, so we're going to lint about that.

This is a bit of an odd lint for `rustc`.  We normally don't lint just
to have people confirm that they understand how Rust works.  But in
this one exceptional case, this seems like the right thing to do as
compared to the other plausible alternatives.

In this commit, we describe the nature of this odd lint.

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