elaborate on Timer docs (#6385)

These tiny changes answer question I had when using the Timer class.

Author: hmeine

crates/bevy_time/src/timer.rs

@@ -47,7 +47,8 @@ impl Timer {
         }
     }
 
-    /// Returns `true` if the timer has reached its duration.
+    /// Returns `true` if the timer has reached its duration at least once.
+    /// See also [`Timer::just_finished`](Timer::just_finished).
     ///
     /// # Examples
     /// ```
@@ -188,6 +189,7 @@ impl Timer {
     /// Advance the timer by `delta` seconds.
     /// Non repeating timer will clamp at duration.
     /// Repeating timer will wrap around.
+    /// Will not affect paused timers.
     ///
     /// See also [`Stopwatch::tick`](Stopwatch::tick).
     ///