Adds doc note that Timer and Stopwatch must be progressed manually (#…

…13441)

# Objective
- Fix #13421 

## Solution

- Add an explicit note at the root of each struct that they must be
ticked manually

## Testing

- Generated the docs and the changes look good
---

---------

Co-authored-by: François Mockers <francois.mockers@vleue.com>
Co-authored-by: Matty <weatherleymatthew@gmail.com>

crates/bevy_time/src/stopwatch.rs

@@ -4,6 +4,7 @@ use bevy_utils::Duration;
 
 /// A Stopwatch is a struct that track elapsed time when started.
 ///
+/// Note that in order to advance the stopwatch [`tick`](Stopwatch::tick) **MUST** be called.
 /// # Examples
 ///
 /// ```

crates/bevy_time/src/timer.rs

@@ -10,6 +10,8 @@ use bevy_utils::Duration;
 /// exceeded, and can still be reset at any given point.
 ///
 /// Paused timers will not have elapsed time increased.
+///
+/// Note that in order to advance the timer [`tick`](Timer::tick) **MUST** be called.
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 #[cfg_attr(feature = "serialize", derive(serde::Deserialize, serde::Serialize))]
 #[cfg_attr(feature = "bevy_reflect", derive(Reflect), reflect(Default))]