Add trait bounds to glossary (google#1569)

Hi, this is a tiny MR to add glossary entry for "trait bound". Could you
someone review it? I'm open to suggeston, too. Thank you!
Context:
google#1436 (comment)

Author: kantasv

src/glossary.md

@@ -149,6 +149,8 @@ Also, please keep the hard line breaks to ensure a nice formatting.
   The property of a program that ensures correct behavior in a multithreaded environment.
 - trait:\
   A collection of methods defined for an unknown type, providing a way to achieve polymorphism in Rust.
+- trait bound:\
+  An abstraction where you can require types to implement some traits of your interest.
 - type:\
   A classification that specifies which operations can be performed on values of a particular kind in Rust.
 - type inference:\