Document `impl` trait (RFC #1522, RFC #1951, RFC #2250)

[matthewjasper]

This is stabilising with rust-lang/rust#49255 in 1.26. I will probably add this after #258 has merged.

[Havvy]

Are you still (planning on) working on this?

[matthewjasper]

Yes, but it might take me some time to get to this.

[Havvy]

From Discord:

centril: impl Trait should really be considered an inference variable that hides the type identity behind an opaque interface
centril: it's similar to _ in this regard