You don't have to - its in the handbook.
When I got my first service last year, Lexus Guildord modified the auto-locking so that it auto unlocks when you shift into park. That way when you drop off a passanger there is no need to push the unlock button and your passanger isn't left struggling with a locked door. I would recommend this.
With regards to disabling the feature yourself, there are instructions in the manual but I was told by Lexus that it doesn't work on UK Lexus models and it didn't work on mine when I tried it.
Hope this helps.