docs: AnimationPlayer::play doesn't have transition_duration arg (#10970)

# Objective

The documentation for `AnimationPlayer::play` mentions a non-existent
`transition_duration` argument from an old iteration of the API. It's
confusing.

## Solution

Remove the offending sentence.

crates/bevy_animation/src/lib.rs

@@ -285,8 +285,6 @@ impl AnimationPlayer {
     }
 
     /// Start playing an animation, resetting state of the player, unless the requested animation is already playing.
-    /// If `transition_duration` is set, this will use a linear blending
-    /// between the previous and the new animation to make a smooth transition
     pub fn play(&mut self, handle: Handle<AnimationClip>) -> &mut Self {
         if !self.is_playing_clip(&handle) || self.is_paused() {
             self.start(handle);