Add nodoc to Arel filter classes.

- this makes it consistent with other Arel files

Author: simi

activerecord/lib/arel/filter_predications.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-module Arel
+module Arel # :nodoc: all
   module FilterPredications
     def filter(expr)
       Nodes::Filter.new(self, expr)

activerecord/lib/arel/nodes/filter.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-module Arel
+module Arel # :nodoc: all
   module Nodes
     class Filter < Binary
       include Arel::WindowPredications