removed TIP: :allow_nil/:allow_blank is ignored by the presence valid…

…ator

These tips were documenting an inconsistency issue rails#8621. That issue is resolved by rails#8622.

guides/source/active_record_validations.md

@@ -656,8 +656,6 @@ class Coffee < ActiveRecord::Base
 end
 ```
 
-TIP: `:allow_nil` is ignored by the presence validator.
-
 ### `:allow_blank`
 
 The `:allow_blank` option is similar to the `:allow_nil` option. This option
@@ -673,8 +671,6 @@ Topic.create("title" => "").valid?  # => true
 Topic.create("title" => nil).valid? # => true
 ```
 
-TIP: `:allow_blank` is ignored by the presence validator.
-
 ### `:message`
 
 As you've already seen, the `:message` option lets you specify the message that