Associated type bounds need documentation

[Mark-Simulacrum]

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https://github.com/rust-lang/rust/pull/122055/ stabilized associated type bounds, but does not appear to have been accompanied by documentation reflecting the details. We will need to persist that into the reference.

[ehuss]

Hm, we added some basic grammar in #1481, but I probably should have pushed to have more documentation added to the trait bounds docs. Those docs seem pretty light, though, so I'm not sure exactly what should be added without going down the rabbit hole of making the docs more explicit.