Change headers in Traits section of the book

src/doc/trpl/glossary.md

@@ -38,6 +38,14 @@ let z = (8, 2, 6);
 
 In the example above `x` and `y` have arity 2. `z` has arity 3.
 
+### Bounds
+
+Bounds are constraints on a type or [trait][traits]. For example, if a bound
+is placed on the argument a function takes, types passed to that function
+must abide by that constraint.
+
+[traits]: traits.html
+
 ### DST (Dynamically Sized Type)
 
 A type without a statically known size or alignment. ([more info][link])

src/doc/trpl/traits.md

@@ -47,12 +47,14 @@ As you can see, the `trait` block looks very similar to the `impl` block,
 but we don’t define a body, just a type signature. When we `impl` a trait,
 we use `impl Trait for Item`, rather than just `impl Item`.
 
-## Traits bounds for generic functions
+## Trait bounds on generic functions
 
 Traits are useful because they allow a type to make certain promises about its
-behavior. Generic functions can exploit this to constrain the types they
+behavior. Generic functions can exploit this to constrain, or [bound][bounds], the types they
 accept. Consider this function, which does not compile:
 
+[bounds]: glossary.html#bounds
+
 ```rust,ignore
 fn print_area<T>(shape: T) {
     println!("This shape has an area of {}", shape.area());
@@ -66,7 +68,7 @@ error: no method named `area` found for type `T` in the current scope
 ```
 
 Because `T` can be any type, we can’t be sure that it implements the `area`
-method. But we can add a ‘trait constraint’ to our generic `T`, ensuring
+method. But we can add a trait bound to our generic `T`, ensuring
 that it does:
 
 ```rust
@@ -155,10 +157,10 @@ We get a compile-time error:
 error: the trait `HasArea` is not implemented for the type `_` [E0277]
 ```
 
-## Traits bounds for generic structs
+## Trait bounds on generic structs
 
-Your generic structs can also benefit from trait constraints. All you need to
-do is append the constraint when you declare type parameters. Here is a new
+Your generic structs can also benefit from trait bounds. All you need to
+do is append the bound when you declare type parameters. Here is a new
 type `Rectangle<T>` and its operation `is_square()`:
 
 ```rust