updating description of how class_attribute works

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -18,6 +18,11 @@ class Class
   #   Subclass.setting            # => false
   #   Base.setting                # => true
   #
+  #   In the above case as long as Subclass does not assign a value to setting
+  #   by performing <tt>Subclass.setting = _something_ </tt>, <tt>Subclass.setting</tt>
+  #   would read value assigned to parent class. Once Subclass assigns a value then
+  #   the value assigned by Subclass would be returned.
+  #
   # This matches normal Ruby method inheritance: think of writing an attribute
   # on a subclass as overriding the reader method.
   #