Update documentation on group method to indicate it takes an array

The group method also takes an array, however this isn't immediately
clear by reading the source since it delegates this method. If you
trace it back to the AREL building you can see that it does support
an array.

Shoutout to @betovelandia for pointing this out.

activerecord/lib/active_record/relation/query_methods.rb

@@ -261,6 +261,10 @@ def select!(*fields) # :nodoc:
     #
     #   User.group('name AS grouped_name, age')
     #   => [#<User id: 3, name: "Foo", age: 21, ...>, #<User id: 2, name: "Oscar", age: 21, ...>, #<User id: 5, name: "Foo", age: 23, ...>]
+    #
+    # Passing in an array of attributes to group by is also supported.
+    #   User.select([:id, :first_name]).group(:id, :first_name).first(3)
+    #   => [#<User id: 1, first_name: "Bill">, #<User id: 2, first_name: "Earl">, #<User id: 3, first_name: "Beto">]
     def group(*args)
       check_if_method_has_arguments!(:group, args)
       spawn.group!(*args)