Update old docs from Timer (#6646)

When I was upgrading to 0.9 noticed there were some changes to the timer, mainly the `TimerMode`.  When switching from the old `is_repeating()`  and `set_repeating()` to the new `mode()` and `set_mode()` noticed the docs still had the old description.

crates/bevy_time/src/timer.rs

@@ -155,7 +155,7 @@ impl Timer {
  self.duration = duration;
  }
 
- /// Returns `true` if the timer is repeating.
+ /// Returns the mode of the timer.
  ///
  /// # Examples
  /// ```
@@ -168,7 +168,7 @@ impl Timer {
  self.mode
  }
 
- /// Sets whether the timer is repeating or not.
+ /// Sets the mode of the timer.
  ///
  /// # Examples
  /// ```
@@ -177,6 +177,7 @@ impl Timer {
  /// timer.set_mode(TimerMode::Once);
  /// assert_eq!(timer.mode(), TimerMode::Once);
  /// ```
+ #[doc(alias = "repeating")]
  #[inline]
  pub fn set_mode(&mut self, mode: TimerMode) {
  if self.mode != TimerMode::Repeating && mode == TimerMode::Repeating && self.finished {