title: Firestore onSnapshot -> Does it work for subcollections?
body: <p>Is the method onSnapshot of firestore triggered by any changes in subcollections?</p>
  
  <p>What is the difference between 
  db.firestore.collection('x'). onSnapshot</p>
  
  <p>And db.firestore.collection('x').doc('y').collection('z'). onSnapshot</p>
  
  <p>Are they equal????</p>