nomicon: Mention contravariance

Author: bluss

src/doc/nomicon/subtyping.md

@@ -44,10 +44,11 @@ subtyping of its outputs. There are two kinds of variance in Rust:
 * F is *invariant* over `T` otherwise (no subtyping relation can be derived)
 
 (For those of you who are familiar with variance from other languages, what we
-refer to as "just" variance is in fact *covariance*. Rust does not have
-contravariance. Historically Rust did have some contravariance but it was
-scrapped due to poor interactions with other features. If you experience
-contravariance in Rust call your local compiler developer for medical advice.)
+refer to as "just" variance is in fact *covariance*. Rust has *contravariance*
+for functions. The future of contravariance is uncertain and it may be
+scrapped. For now, `fn(T)` is contravariant in `T`, which is used in matching
+methods in trait implementations to the trait definition. Traits don't have
+inferred variance, so `Fn(T)` is invariant in `T`).
 
 Some important variances: