Add a comment for height change functions about the step field

Changing the height is a light action. Height change functions change
all fields in Tendermint Worker except the `step` field.

`move_to_step` function changes the `step` field and does many other
things, such as sending the current state to other peers, requesting
other peers regarding votes, starting a timer, etc.

The side-effect of the `move_to_step` function is that the
`move_to_the_next_height` function and the `jump_to_height` function
do not call `move_to_step`. The caller of the height change functions
should call the `move_tp_step` function.

Author: Juhyung Park

core/src/consensus/tendermint/worker.rs

@@ -628,6 +628,8 @@ impl Worker {
     }
 
     /// Move to the next height.
+    /// Since changing the `step` needs many things to do, this function does not change the `step` variable.
+    /// The caller should call `move_to_step` after calling this function.
     fn move_to_the_next_height(&mut self) {
         assert!(
             self.step.is_commit(),
@@ -648,6 +650,8 @@ impl Worker {
     /// Jump to the height.
     /// This function is called when new blocks are received from block sync.
     /// This function could be called at any state.
+    /// Since changing the `step` needs many things to do, this function does not change the `step` variable.
+    /// The caller should call `move_to_step` after calling this function.
     fn jump_to_height(&mut self, height: Height, finalized_view_of_previous_height: View) {
         assert!(height > self.height, "{} < {}", height, self.height);
         cinfo!(ENGINE, "Transitioning to height {}.", height);