Update docs for image() by J16053 · Pull Request #5950 · processing/processing

This is my first time contributing here, so please let me know if there's anything I can do better.

I was looking through the Processing docs to find the info for a function that I saw someone use in their code, but was surprised to find that the cropping capability using the image() function had no documentation. So this is me trying to include it! If this looks good, I'll also be opening a pull request in the processing/processing-docs repo to update the description in the xml.