Add more examples to #in_order_of [ci-skip]

Adds more examples to `#in_order_of`:
- what happens when dealing with `enum` +columns+
- what happens when passing `nil` as a +value+ for nullable columns

Author: thdaraujo

activerecord/lib/active_record/relation/query_methods.rb

@@ -606,7 +606,8 @@ def order!(*args) # :nodoc:
       self
     end
 
-    # Allows to specify an order by a specific set of values.
+    # Allows to specify an <code>ORDER BY</code> clause to a query based on a given +column+,
+    # ordered and filtered by the specific set of +values+.
     #
     #   User.in_order_of(:id, [1, 5, 3])
     #   # SELECT "users".* FROM "users"
@@ -617,6 +618,32 @@ def order!(*args) # :nodoc:
     #   #     WHEN "users"."id" = 3 THEN 3
     #   #   END ASC
     #
+    # +column+ can point to an enum column; the actual query generated may be different depending
+    # on the database adapter and the column definition.
+    #
+    #   class Conversation < ActiveRecord::Base
+    #     enum :status, [ :active, :archived ]
+    #   end
+    #
+    #   Conversation.in_order_of(:status, %w(archived active))
+    #   # SELECT "conversations".* FROM "conversations"
+    #   #   WHERE "conversations"."status" IN (1, 0)
+    #   #   ORDER BY CASE
+    #   #     WHEN "conversations"."status" = 1 THEN 1
+    #   #     WHEN "conversations"."status" = 0 THEN 2
+    #   #   END ASC
+    #
+    # +values+ can also include +nil+.
+    #
+    #   Conversation.in_order_of(:status, %w(nil archived active))
+    #   # SELECT "conversations".* FROM "conversations"
+    #   #   WHERE ("conversations"."status" IN (1, 0) OR "conversations"."status" IS NULL)
+    #   #   ORDER BY CASE
+    #   #     WHEN "conversations"."status" IS NULL THEN 1
+    #   #     WHEN "conversations"."status" = 1 THEN 2
+    #   #     WHEN "conversations"."status" = 0 THEN 3
+    #   #   END ASC
+    #
     def in_order_of(column, values)
       klass.disallow_raw_sql!([column], permit: connection.column_name_with_order_matcher)
       return spawn.none! if values.empty?