[DOC] Mention the omission of a superclass when reopening a class

doc/syntax/modules_and_classes.rdoc

@@ -40,9 +40,9 @@ functionality:
     remove_method :my_method
   end
 
-Reopening classes is a very powerful feature of Ruby, but it is best to only
+Reopening modules (or classes) is a very powerful feature of Ruby, but it is
-reopen classes you own.  Reopening classes you do not own may lead to naming
+best to only reopen modules you own.  Reopening modules you do not own may lead
-conflicts or difficult to diagnose bugs.
+to naming conflicts or difficult to diagnose bugs.
 
 == Nesting
 
@@ -259,6 +259,28 @@ includes a minimum of built-in methods.  You can use BasicObject to create an
 independent inheritance structure.  See the BasicObject documentation for
 further details.
 
+Just like modules, classes can also be reopened.  You can omit its superclass
+when you reopen a class.  Specifying a different superclass than the previous
+definition will raise an error.
+
+  class C
+  end
+
+  class D < C
+  end
+
+  # OK
+  class D < C
+  end
+
+  # OK
+  class D
+  end
+
+  # TypeError: superclass mismatch for class D
+  class D < String
+  end
+
 == Inheritance
 
 Any method defined on a class is callable from its subclass: