docs: document is operator as overridable infix method and add type-test examples

doc/site/core/object.markdown

@@ -3,7 +3,7 @@
 
 ## Static Methods
 
-## **same**(obj1, obj2)
+### **same**(obj1, obj2)
 
 Returns `true` if *obj1* and *obj2* are the same. For [value
 types](../values.html), this returns `true` if the objects have equivalent
@@ -20,7 +20,9 @@ even on user-defined classes.
 
 ### **!** operator
 
-Returns `false`, since most objects are considered [true](control-flow.html#truth).
+Returns `false`, since most objects are considered [true][].
+
+[true]: control-flow.html#truth
 
 ### **==**(other) and **!=**(other) operators
 
@@ -28,10 +30,25 @@ Compares two objects using built-in equality. This compares [value
 types](../values.html) by value, and all other objects are compared by
 identity—two objects are equal only if they are the exact same object.
 
+### **is**(class) operator
+
+Returns `true` if this object's class or one of its superclasses is `class`.
+
+    :::wren
+    System.print(123 is Num)     //> true
+    System.print("s" is Num)     //> false
+    System.print(null is String) //> false
+    System.print([] is List)     //> true
+    System.print([] is Sequence) //> true
+
+It is a runtime error if `class` is not a [Class][].
+
 ### **toString**
 
 A default string representation of the object.
 
 ### **type**
 
-The [Class](#class-class) of the object.
+The [Class][] of the object.
+
+[class]: class.html