type inference chapter should talk about opaque types

[oli-obk]

Opaque types (impl Trait) uses inference variables in several situations. We should explain how that works (with examples!), both for impl trait in return position and type alias impl trait.

[smanilov]

Not only do I think this chapter has been written since the issue was opened... but I think it's even outdated :D (It's how I found this issue.)

Here's the chapter: https://rustc-dev-guide.rust-lang.org/opaque-types-impl-trait-inference.html

I think we can close this one, and open another one to track the updating of the chapter. Or maybe we can just track it here. Here's one PR towards that goal: #2429

[smanilov]

Also #2435

[smanilov]

In my opinion as a noob, it seems that there is no more updating of the chapter necessary at this point. Its quality is of acceptable level, when compared to the rest of the documentation.

[oli-obk]

Thanks for looking into this!