message: comments on upcall_on_command field
diff: @@ -9,6 +9,9 @@ use crate::{DriverInfo, DriverShareRef};
  use libtock_platform::{CommandReturn, ErrorCode};
  use std::cell::Cell;
  +// The `upcall_on_command` field is set to Some(value) if an upcall(with value as its argument) should be called when read command is received,
  +// or None otherwise. It was needed for testing `read_sync` library function which simulates a synchronous temperature read,
  +// because it was impossible to schedule an upcall during the `synchronous` read in other ways.
  pub struct Temperature {
  busy: Cell<bool>,
  upcall_on_command: Cell<Option<i32>>,
  