language tour: improve Traits page

fixes #3097

Author: SethTisue

_tour/traits.md

@@ -12,9 +12,14 @@ prerequisite-knowledge: expressions, classes, generics, objects, companion-objec
 redirect_from: "/tutorials/tour/traits.html"
 ---
 
-Traits are used to share interfaces and fields between classes. They are similar to Java 8's interfaces. Classes and objects can extend traits, but traits cannot be instantiated and therefore have no parameters.
+Traits are used to represent shared aspects of multiple types. They are similar to Java's interfaces and to what some languages call "mixins".
+
+A trait may contain member `def`s, `val`s, and `var`s, which may be abstract or concrete. "Concrete" means the trait includes an implementation for that member.
+
+A trait can be extended by classes, objects, and other traits. Extending multiple traits is allowed. A trait may extend a class.
 
 ## Defining a trait
+
 A minimal trait is simply the keyword `trait` and an identifier:
 
 {% tabs trait-hair-color %}
@@ -158,6 +163,18 @@ animals.foreach(pet => println(pet.name))  // Prints Harry Sally
 
 The `trait Pet` has an abstract field `name` that gets implemented by Cat and Dog in their constructors. On the last line, we call `pet.name`, which must be implemented in any subtype of the trait `Pet`.
 
+## Trait parameters
+
+In Scala 3, a trait may accept constructor parameters:
+
+{% tabs trait-parameter %}
+{% tab 'Scala 3' for=trait-parameter %}
+```scala mdoc
+trait HasLegs(legCount: Int)
+class Spider extends HasLegs(8)
+```
+{% endtab %}
+{% endtabs %}
 
 ## More resources