You say not to warn if we are not willing to actually follow through when setting boundaries, but that does not work long term. So is it never a good idea to use this way of discipline? For example, "Mommy, can I eat a chocolate? Yes, but only after dinner. And you need to actually eat your dinner, otherwise there's no chocolate." Or "You can watch something on the phone for x amount of time," then time runs out (I usually give her notice before time is running out) and she doesn't want to stop watching or give the phone back. After asking a few times, I would say, "Mommy said you had x amount of time, now time ran out. Please give me back the phone or I'm going to have to take it from you" or "You can give me the phone back or I can get it from you." Are these all considered punishment and rewards? Also, how about the counting method? Is it also bad news?
You gave a great example of giving the "how and why" and the rule is concise, informative and to the point. So keep using this-- The example with managing the phone and what you state to her is also a good way to express a boundary and rule. The only tweak to the phone example is when time is up you remove the phone and like we say in the call #embracethetantrum. Be kind when removing it, which you are doing by saying -- "You can give me the phone back or I can get it from you." The key is to say this phrase, and then take the phone. When you embrace the tantrum and end the time the child is using the toy, phone, TV, etc- this is when you are helping them delay gratification and tolerate their impulses/build impulse control. These are highly important skills in school and in social interactions. Having her feel the frustration of not being able to have her pleasure is necessary at this phase of development.
The examples you gave are great and not examples of punishments and rewards, about which I speak often. A punishment phrase is "if you don't stop using the phone right now I am going to take away all your toys, not let you eat dessert, will tell your daddy you' are bad, etc." That phrase has nothing to do with the task at hand, the rule, or gives any direction on how to handle her impulsivity. An example of a reward phrase is, "if you give me the phone back mommy will give you a cookie" Again this has nothing to do with the task or the lesson of delaying gratification. With this reward you are replacing one pleasure for the other.