Docs: Clarify what user API search searches for

doc/api/users.md

@@ -39,7 +39,7 @@ GET /users
 ]
 ```
 
-You can also search for users by name or primary email using `?search=`. For example. `/users?search=John`.
+You can also search for users by name, username, primary email, or secondary email, by using `?search=`. For example. `/users?search=John`.
 
 In addition, you can lookup users by username: