ImageSource::Bytes docs should mention URIs are used for format detection

[xTibor]

Describe the bug
The new ImageSource::Bytes docs does not mention providing URIs are mandatory and used for format detection. This is kinda a significant detail.

Based on the current docs one might think these URIs are there just for error reporting and egui is using some kind of format probing on the provided data instead. At least that's what I first thought trying out the new API.

[emilk]

Good point - will fix momentarily