Clarify usage of AnimationPlayer with AnimationTree and fill in empty method descriptions
Clarify usage of AnimationPlayer with AnimationTree and fill in empty method descriptions

Add clarification to docs to explain that when an `AnimationPlayer`
object is paired with an `AnimationTree`, several properties and
methods exposed by the `AnimationPlayer` class may not work as expected.
The issues occur because an `AnimationTree` runs independently, and
uses its member `AnimationPlayer* player` primarily for its
`Map<StringName, AnimationData> animation_set`.

Added descriptions for `root_motion_track` and `get_root_motion_transform`.

Shockblast/godot,akien-mga/godot,godotengine/godot,MarianoGnu/godot,firefly2442/godot,firefly2442/godot,Paulloz/godot,Paulloz/godot,godotengine/godot,honix/godot,vkbsb/godot,josempans/godot,josempans/godot,ZuBsPaCe/godot,Shockblast/godot,josempans/godot,honix/godot,josempans/godot,guilhermefelipecgs/godot,firefly2442/godot,BastiaanOlij/godot,Faless/godot,ZuBsPaCe/godot,Zylann/godot,Paulloz/godot,Zylann/godot,sanikoyes/godot,vnen/godot,Faless/godot,akien-mga/godot,Faless/godot,vnen/godot,vnen/godot,Paulloz/godot,Valentactive/godot,MarianoGnu/godot,vnen/godot,josempans/godot,Valentactive/godot,DmitriySalnikov/godot,vkbsb/godot,Zylann/godot,akien-mga/godot,DmitriySalnikov/godot,Shockblast/godot,Shockblast/godot,Zylann/godot,Valentactive/godot,godotengine/godot,Zylann/godot,sanikoyes/godot,Shockblast/godot,firefly2442/godot,pkowal1982/godot,BastiaanOlij/godot,ZuBsPaCe/godot,vnen/godot,Faless/godot,DmitriySalnikov/godot,ZuBsPaCe/godot,vkbsb/godot,vkbsb/godot,ZuBsPaCe/godot,honix/godot,Faless/godot,akien-mga/godot,josempans/godot,DmitriySalnikov/godot,vkbsb/godot,pkowal1982/godot,DmitriySalnikov/godot,honix/godot,pkowal1982/godot,pkowal1982/godot,ZuBsPaCe/godot,sanikoyes/godot,Shockblast/godot,sanikoyes/godot,godotengine/godot,sanikoyes/godot,guilhermefelipecgs/godot,vnen/godot,guilhermefelipecgs/godot,MarianoGnu/godot,Zylann/godot,DmitriySalnikov/godot,BastiaanOlij/godot,DmitriySalnikov/godot,Faless/godot,Valentactive/godot,BastiaanOlij/godot,guilhermefelipecgs/godot,pkowal1982/godot,firefly2442/godot,BastiaanOlij/godot,ZuBsPaCe/godot,firefly2442/godot,ZuBsPaCe/godot,guilhermefelipecgs/godot,godotengine/godot,honix/godot,Shockblast/godot,MarianoGnu/godot,godotengine/godot,BastiaanOlij/godot,josempans/godot,guilhermefelipecgs/godot,Valentactive/godot,Zylann/godot,firefly2442/godot,akien-mga/godot,Faless/godot,vkbsb/godot,vkbsb/godot,godotengine/godot,akien-mga/godot,Paulloz/godot,MarianoGnu/godot,akien-mga/godot,pkowal1982/godot,godotengine/godot,honix/godot,MarianoGnu/godot,Zylann/godot,Valentactive/godot,Valentactive/godot,pkowal1982/godot,pkowal1982/godot,sanikoyes/godot,josempans/godot,BastiaanOlij/godot,Shockblast/godot,vnen/godot,firefly2442/godot,Valentactive/godot,akien-mga/godot,BastiaanOlij/godot,Paulloz/godot,MarianoGnu/godot,sanikoyes/godot,MarianoGnu/godot,Paulloz/godot,Faless/godot,guilhermefelipecgs/godot,guilhermefelipecgs/godot,vkbsb/godot,vnen/godot,sanikoyes/godot