[runtimes] Ensure all language runtimes have up-to-date code documentation

[x] spine-libgdx

[ ] spine-csharp
[ ] spine-xna
[ ] spine-monogame
[ ] spine-unity
[ ] spine-tk2d

[ ] spine-c
[ ] spine-cpp
[ ] spine-sfml
[ ] spine-cocos2d-objc
[ ] spine-cocos2dx
[ ] spine-ue4

[ ] spine-ts core
[ ] spine-ts webgl
[ ] spine-ts canvas
[ ] spine-ts threejs

[ ] spine-as3
[ ] spine-starling

[ ] spine-lua
[ ] spine-love
[ ] spine-corona

Depending on the version of the runtime, it looks like some of the API has changed with very similar functionality but some slight differences. I don't understand enough about how Attachments, Skins, Bones, etc interact to immediately know what the differences are. Before I dive into some research, I thought I'd make an issue here to see if there's already documentation in place for this type of thing, or if it might be worth putting some notes in place for people in my situation going forward in the future.

Here is the only source of API docs I could find, with no obvious way to reference historical versions: http://esotericsoftware.com/spine-runtime-skins

Specifically, I'm having trouble with the Skins API. It has changed a bit from 3.7 to 3.8 to 3.9.

In this case, using the spine-ts runtime, I can't tell what the differences between Skin.addSkin() and Skin.copySkin() are doing. Both methods seem to work for interchangeably for me when I do something like

// findSkin is a wrapper for _spineGO.skeletonData.findSkin(skinName);
const skin: Skin = this._spineGO.findSkin("body_001");
const face: Skin = this._spineGO.findSkin("face_001");
const top: Skin = this._spineGO.findSkin("top_002");
const bottom: Skin = this._spineGO.findSkin("bottom_001");
const weapon: Skin = this._spineGO.findSkin("weapon_001");

skin.copySkin(face);
skin.addSkin(top);
skin.copySkin(bottom);
skin.copySkin(weapon);

// blah blah
// setSkin(skin)

EsotericSoftware / spine-runtimes

[runtimes] Ensure all language runtimes have up-to-date code documentation #1506