add on_real_time_timer run condition (bevyengine#10179)

# Objective

Fixes bevyengine#10177 .

## Solution

Added a new run condition and tweaked the docs for `on_timer`.

## Changelog

### Added 
 - `on_real_time_timer` run condition

crates/bevy_time/src/common_conditions.rs

@@ -1,9 +1,9 @@
-use crate::{Time, Timer, TimerMode};
+use crate::{Real, Time, Timer, TimerMode};
 use bevy_ecs::system::Res;
 use bevy_utils::Duration;
 
 /// Run condition that is active on a regular time interval, using [`Time`] to advance
-/// the timer.
+/// the timer. The timer ticks at the rate of [`Time::relative_speed`].
 ///
 /// ```rust,no_run
 /// # use bevy_app::{App, NoopPluginGroup as DefaultPlugins, PluginGroup, Update};
@@ -38,6 +38,42 @@ pub fn on_timer(duration: Duration) -> impl FnMut(Res<Time>) -> bool + Clone {
     }
 }
 
+/// Run condition that is active on a regular time interval, using [`Time<Real>`] to advance
+/// the timer. The timer ticks are not scaled.
+///
+/// ```rust,no_run
+/// # use bevy_app::{App, NoopPluginGroup as DefaultPlugins, PluginGroup, Update};
+/// # use bevy_ecs::schedule::IntoSystemConfigs;
+/// # use bevy_utils::Duration;
+/// # use bevy_time::common_conditions::on_real_timer;
+/// fn main() {
+///     App::new()
+///         .add_plugins(DefaultPlugins)
+///         .add_systems(Update, tick.run_if(on_real_timer(Duration::from_secs(1))))
+///         .run();
+/// }
+/// fn tick() {
+///     // ran once a second
+/// }
+/// ```
+///
+/// Note that this does **not** guarantee that systems will run at exactly the
+/// specified interval. If delta time is larger than the specified `duration` then
+/// the system will only run once even though the timer may have completed multiple
+/// times. This condition should only be used with large time durations (relative to
+/// delta time).
+///
+/// For more accurate timers, use the [`Timer`] class directly (see
+/// [`Timer::times_finished_this_tick`] to address the problem mentioned above), or
+/// use fixed timesteps that allow systems to run multiple times per frame.
+pub fn on_real_timer(duration: Duration) -> impl FnMut(Res<Time<Real>>) -> bool + Clone {
+    let mut timer = Timer::new(duration, TimerMode::Repeating);
+    move |time: Res<Time<Real>>| {
+        timer.tick(time.delta());
+        timer.just_finished()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;