image of text: clarify why pictures, but not all images with text and other significant content are exempt [0va7u6]

[WilcoFiers]

We had some feedback from ag on this definition. I think our interpretation of it is correct, but we should clarify we've made an assumption in the rule about how to interpret the word "picture":

image of text: text that has been rendered in a non-text form (e.g., an image) in order to achieve a particular visual effect

NOTE: This does not include text that is part of a picture that contains significant other visual content.