Update virtual_type.md

This takes the better manual documentation from the nit website.

doc/manual/virtual_type.md

@@ -1,20 +1,48 @@
 # Virtual Types
 
-`type` declares a virtual types in a class. A bound type is
-mandatory. Virtual types can then be used as regular types in the class
-and its subclasses. Subclasses can also redefine it with a more specific
-bound type. One can see a virtual type as an internal formal generic
-parameter or as a redefinable *typedef*.
+Virtual type is a way to define in a class a property that is associated with a type.
+This property can be used to type parameters, return of functions, variables, etc.
+Virtual types are inherited by subclasses and can be redefined.
 
-~~~
-class Foo
-    type E: Object
-    var derp: E
+Virtual types is a good solution when:
+
+  * A class need to refer to the real itself
+  * Two class hierarchy are somewhat parallel
+  * You want covariance
+
+### Usage
+
+Example, we have employees who work in standard office and bosses, that are technically employees, but they work in boss offices (that are improved offices with a fridge).
+
+One way to model this is to use virtual types.
+Virtual types are defined inside class (like methods and attributes) but with the keyword `type`.
+Inside the class, the virtual type can be used (almost) like any other static type.
+
+Subclasses can redefine (with `redef`) the virtual types they want to change.
+The only requirement is that the new bound is a sub-type of the previous bound.
+
+The redefinition of a virtual type is used by the compiler to prevent some type errors.
+For example, assuming that boss' offices have a fridge:
+
+~~~nitish
+class Employee
+    type OFFICE: Office
+    var office: OFFICE
+end
+class Office
+    # ...
 end
-class Bar
-    super Foo
-    redef type E: Int
+
+class Boss
+    super Employee
+    redef type OFFICE: BossOffice
+end
+class BossOffice
+    super Office
 end
-var b = new Bar(5)
-print b.derp + 1 # outputs 6
+
+var e: Employee = ...
+e.office.fridge.open # Compilation Error! Office has no method fridge
+var b: Boss = ...
+b.office.fridge.open # OK!
 ~~~