move the tips about :allow_(nil|blank) near their explanations, also …

…no need to say these are options for all validators if they are not

railties/guides/source/active_record_validations_callbacks.textile

@@ -461,9 +461,7 @@ The block receives the model, the attribute's name and the attribute's value. Yo
 
 h3. Common Validation Options
 
-There are some common options that all the validation helpers can use. Here they are, except for the +:if+ and +:unless+ options, which are discussed later in "Conditional Validation":#conditional-validation.
-
-TIP: Note that +allow_nil+ and +allow_blank+ will be ignored when using the presence validator. Please use the length validator if you want to validate if something is a specific length but allows for +nil+ values.
+These are common validation options:
 
 h4. +:allow_nil+
 
@@ -476,6 +474,8 @@ class Coffee < ActiveRecord::Base
 end
 </ruby>
 
+TIP: +:allow_nil+ is ignored by the presence validator.
+
 h4. +:allow_blank+
 
 The +:allow_blank+ option is similar to the +:allow_nil+ option. This option will let validation pass if the attribute's value is +blank?+, like +nil+ or an empty string for example.
@@ -489,6 +489,8 @@ Topic.create("title" => "").valid? # => true
 Topic.create("title" => nil).valid? # => true
 </ruby>
 
+TIP: +:allow_blank+ is ignored by the presence validator.
+
 h4. +:message+
 
 As you've already seen, the +:message+ option lets you specify the message that will be added to the +errors+ collection when validation fails. When this option is not used, Active Record will use the respective default error message for each validation helper.