Fix code comments

Author: johnnyshields

lib/mongoid/config.rb

@@ -317,8 +317,8 @@ def running_with_passenger?
     # Defines a field type mapping, for later use in field :type option.
     #
     # @example
-    #   Mongoid.configure do
-    #     define_field_type :point, Point
+    #   Mongoid.configure do |config|
+    #     config.define_field_type :point, Point
     #   end
     #
     # @param [ Symbol | String ] type_name The identifier of the
@@ -338,8 +338,8 @@ def define_field_type(type_name, klass)
     # provided in the field definition -- even if it is false or nil.
     #
     # @example
-    #   Mongoid.configure do
-    #     define_field_option :required do |model, field, value|
+    #   Mongoid.configure do |config|
+    #     config.define_field_option :required do |model, field, value|
     #       model.validates_presence_of field.name if value
     #     end
     #   end

lib/mongoid/fields.rb

@@ -265,10 +265,8 @@ class << self
       # provided in the field definition -- even if it is false or nil.
       #
       # @example
-      #   Mongoid::Fields.configure do
-      #     option :required do |model, field, value|
-      #       model.validates_presence_of field.name if value
-      #     end
+      #   Mongoid::Fields.option :required do |model, field, value|
+      #     model.validates_presence_of field.name if value
       #   end
       #
       # @param [ Symbol ] option_name the option name to match against