Make arel methods private API

Since its conception arel was made to be private API of Active Record.
If users want to use arel features directly we should provide a way
using the Active Record API without exposing the arel implementation.

activerecord/lib/active_record/core.rb

@@ -138,12 +138,12 @@ module ActiveRecord
       #   class Post < ActiveRecord::Base
       #     scope :published_and_commented, -> { published.and(self.arel_table[:comments_count].gt(0)) }
       #   end
-      def arel_table
+      def arel_table # :nodoc:
         @arel_table ||= Arel::Table.new(table_name, arel_engine)
       end
 
       # Returns the Arel engine.
-      def arel_engine
+      def arel_engine # :nodoc:
         @arel_engine ||=
           if Base == self || connection_handler.retrieve_connection_pool(self)
             self

activerecord/lib/active_record/relation/query_methods.rb

@@ -824,11 +824,12 @@ module ActiveRecord
     end
 
     # Returns the Arel object associated with the relation.
-    def arel
+    def arel # :nodoc:
       @arel ||= build_arel
     end
 
-    # Like #arel, but ignores the default scope of the model.
+    private
+
     def build_arel
       arel = Arel::SelectManager.new(table.engine, table)
 
@@ -854,8 +855,6 @@ module ActiveRecord
       arel
     end
 
-    private
-
     def symbol_unscoping(scope)
       if !VALID_UNSCOPING_VALUES.include?(scope)
         raise ArgumentError, "Called unscope() with invalid unscoping argument ':#{scope}'. Valid arguments are :#{VALID_UNSCOPING_VALUES.to_a.join(", :")}."