Clarify tiling of AtlasTexture

[KoBeWi]

Closes #5660

[arkology]

Is it a documentation issue if tiling worked before (according to #5660 (comment))?

[KoBeWi]

Seems like it works if you set region of the AtlasTexture itself, not Sprite2D or TextureRect.

[KoBeWi]

Pushed additional note.

[Repiteo]

Thanks!

[KoBeWi]

I realized that suggesting region as workaround makes no sense, because it's functional only if you are tiling the whole texture. It won't work if you want to tile part of the texture (e.g. frame in spritesheet).

[Mickeon]

Would we rather revert this and/or not recommend anything?

[KoBeWi]

I'd just remove the last sentence, the other part is correct.
There are supposedly some workarounds, like using shaders, but it's probably too complex to explain here.