Auto merge of #26118 - tshepang:define-trait, r=steveklabnik

I saw a variation of this definition in [an article](http://pcwalton.github.io/blog/2012/08/08/a-gentle-introduction-to-traits-in-rust) by @pcwalton.

Author: bors

src/doc/trpl/traits.md

@@ -1,5 +1,8 @@
 % Traits
 
+A trait is a language feature that tells the Rust compiler about
+functionality a type must provide.
+
 Do you remember the `impl` keyword, used to call a function with [method
 syntax][methodsyntax]?