Make the lack of AtlasTexture support in AnimatedTexture more prominent

See godotengine#33855.

doc/classes/AnimatedTexture.xml

@@ -6,7 +6,8 @@
  <description>
  [AnimatedTexture] is a resource format for frame-based animations, where multiple textures can be chained automatically with a predefined delay for each frame. Unlike [AnimationPlayer] or [AnimatedSprite2D], it isn't a [Node], but has the advantage of being usable anywhere a [Texture2D] resource can be used, e.g. in a [TileSet].
  The playback of the animation is controlled by the [member fps] property as well as each frame's optional delay (see [method set_frame_delay]). The animation loops, i.e. it will restart at frame 0 automatically after playing the last frame.
- [AnimatedTexture] currently requires all frame textures to have the same size, otherwise the bigger ones will be cropped to match the smallest one. Also, it doesn't support [AtlasTexture]. Each frame needs to be separate image.
+ [AnimatedTexture] currently requires all frame textures to have the same size, otherwise the bigger ones will be cropped to match the smallest one.
+ [b]Note:[/b] AnimatedTexture doesn't support using [AtlasTexture]s. Each frame needs to be a separate [Texture2D].
  </description>
  <tutorials>
  </tutorials>