Animation: Document frame duration units in SpriteFrames

[dalexeev]

• Addresses AnimatedSprite{2D,3D} improvements #65609 (comment).

The units are already documented in detail, I think we can just refer to get_frame_duration(), without the reverse formula.

godot/doc/classes/SpriteFrames.xml

Lines 69 to 80 in 71699e0

	<method name="get_frame_duration" qualifiers="const">
	<return type="float" />
	<param index="0" name="anim" type="StringName" />
	<param index="1" name="idx" type="int" />
	<description>
	Returns a relative duration of the frame [param idx] in the [param anim] animation (defaults to [code]1.0[/code]). For example, a frame with a duration of [code]2.0[/code] is displayed twice as long as a frame with a duration of [code]1.0[/code]. You can calculate the absolute duration (in seconds) of a frame using the following formula:
	[codeblock]
	absolute_duration = relative_duration / (animation_fps * abs(playing_speed))
	[/codeblock]
	In this example, [code]playing_speed[/code] refers to either [method AnimatedSprite2D.get_playing_speed] or [method AnimatedSprite3D.get_playing_speed].
	</description>
	</method>

[Mickeon]

Every description around AnimatedSprite could be far improved upon, but it's better than nothing.

[akien-mga]

Thanks!