doc: Add a short AnimationPlayer versus Tween comparison

Many newcomers are confused about which one to choose for animating
properties. This should help clarify the situation with regards
to AnimationPlayer versus Tween.

(cherry picked from commit 810b1341ce)

doc/classes/AnimationPlayer.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		An animation player is used for general-purpose playback of [Animation] resources. It contains a dictionary of animations (referenced by name) and custom blend times between their transitions. Additionally, animations can be played and blended in different channels.
+		[AnimationPlayer] is more suited than [Tween] for animations where you know the final values in advance. For example, fading a screen in and out is more easily done with an [AnimationPlayer] node thanks to the animation tools provided by the editor. That particular example can also be implemented with a [Tween] node, but it requires doing everything by code.
 		Updating the target properties of animations occurs at process time.
 	</description>
 	<tutorials>

doc/classes/Tween.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		Tweens are useful for animations requiring a numerical property to be interpolated over a range of values. The name [i]tween[/i] comes from [i]in-betweening[/i], an animation technique where you specify [i]keyframes[/i] and the computer interpolates the frames that appear between them.
+		[Tween] is more suited than [AnimationPlayer] for animations where you don't know the final values in advance. For example, interpolating a dynamically-chosen camera zoom value is best done with a [Tween] node; it would be difficult to do the same thing with an [AnimationPlayer] node.
 		Here is a brief usage example that causes a 2D node to move smoothly between two positions:
 		[codeblock]
 		var tween = get_node("Tween")