doc: add ! message handler terminator token

doc/sample/Person.im

@@ -49,7 +49,7 @@ class Person
     **/
     - init(name:name age:age)
         self::name = name.
-        self::age = age.
+        self::age = age!
 
     - test(param:data _:extra) | cout put:'Received {data}, {extra}'.
 
@@ -68,16 +68,16 @@ class Person
             #years  => self::age = age,
             #months => self::age = age / 12,
             #days   => self::age = age / 365,
-            _       => self::age = 0,
-        end.
+            _       => self::age = 0
+        end!
 
     - getAgeInUnit:units
         ^match units in
             #years  => self::age,
             #months => self::age / 12,
             #days   => self::age / 365,
-            _       => 0,
-        end.
+            _       => 0
+        end!
 
     /* Properties are defined using the $ symbol.
        They accomplish two things:
@@ -103,9 +103,7 @@ class Person
        is converted to the lambda
             [ :x | self.val = x ]
     */
-    $ exampleProperty
-        get => self.val,
-        set => self.val = value.
+    $ exampleProperty | get => self.val, set => self.val = value.
 
     /* Without the lambda synthesis, the property would look like this:
        Note that this is the only time it is legal to access private fields
@@ -120,8 +118,7 @@ class Person
 
        If either the `set` or `get` elements are not provided, the property
        becomes read-only or write-only respectively */
-    $ exampleProperty2
-        get => 42.
+    $ exampleProperty2 | get => 42.
 
     /* A property can also be configured to act just like a regular variable,
        by setting the getter and setters to default implementations.