docs: clarify Promise.all execution behavior in part4b

[jurajkapsz]

• This update tries to clarify that Promise.all waits for promises to settle rather than executing them, as promises are already running when they are passed to Promise.all (although "settle" might be a new term for students).
• Also clarify that sequential execution is achieved by using await inside a for...of loop, not by merely creating or handling promises inside the loop, such as:

for (const note of helper.initialNotes) {
  new Note(note).save().then(() => {
    // handle success
  });
}