chore(firestore): Clarify Transactioln.Get error code (#6121)

Clarifies that `Transaction.Get()` returns an error with `NotFound` status code.

Co-authored-by: Hidehito Yabuuchi <ordovicia@users.noreply.github.com>
Co-authored-by: Eno Compton <enocom@google.com>

firestore/transaction.go

@@ -195,7 +195,9 @@ func (t *Transaction) rollback() {
 }
 
 // Get gets the document in the context of the transaction. The transaction holds a
-// pessimistic lock on the returned document.
+// pessimistic lock on the returned document. If the document does not exist, Get
+// returns a NotFound error, which can be checked with
+//    status.Code(err) == codes.NotFound
 func (t *Transaction) Get(dr *DocumentRef) (*DocumentSnapshot, error) {
 	docsnaps, err := t.GetAll([]*DocumentRef{dr})
 	if err != nil {