document that loadImage callback is optional

Fixes lmccart/p5js.org#71 .
I am not sure if it is a good idea to document the type as `{Function(p5.Image)}` to kinda show that the callback will return an image. If you like this idea, I can extend it to other functions, so they might look like `{Function(Error,p5.SomeOtherType)}` or `{Function(Number,String)}`. Otherwise I'll just re-send a pull request with just the optional part changed.

Once this one is accepted/rejected, I can send a larger pull request checking which callbacks are optional for the rest of the documentation.