Even those that require you give a certain amount of "notice" before you quit your current job (two_week notice is standard) often may not provide any formal requirement that you do so by writing a letter of resignation or explain in detail why you're leaving.
It also starts the clock for your boss and your company, kicking off the transition plan of training, hiring, and otherwise covering your impending departure. The bad examples here are all too vague. With the last one, you're aligning yourself with company policy, which is good, but it still doesn't tell the reader how much notice that actually is. Be as specific as possible on the time you have between now and your last day.