This adds a section to the file attachment overview docs covering the

ability to switch file attachments on 3.0, showing a screenshot of a PDF

review UI with the navigation buttons present, one expanded to show the

thumbnail.
It also updates the screenshots on the page for the revision selector.

The new screenshot reflects how the UI looks now (instead of the old one

used when the screenshot was first taken), and now only shows one view

of the slider and text instead of several (helping to simplify it so it

doesn't look like there's multiple sliders and making it easier to keep

it up-to-date). This comes with a new Retina version, which the old one

did not have.

Built the docs. Checked for build errors, spelling errors, and

visual errors with the screenshots.