Clarify Premultiplied Alpha

[NoelFB]

One of the biggest issues people immediately run into is rendering with/without premultiplied Alpha. By default the Sprite Batcher uses Premultiplied alpha, and the Color struct multiply operator does the same. I think these assumptions are correct and should be the default, but it needs to be more clear that any images loaded that are not already premultiplied must do so.

I'm not sure if this is just a documentation / FAQ issue, or something that should change in the actual API