Rewrite screenshot review documentation to talk about images.

Review Request #3645 — Created Dec. 9, 2012 and submitted — Latest diff uploaded

Information

Review Board
release-1.7.x

Reviewers

Rewrite screenshot review documentation to talk about images.

I hemmed and hawed a bit about whether this should explain the legacy screenshot
stuff, but going forward, people should see very few of those, so I think it's
better to just cut it entirely. The knowledge they get from the image review
docs should transfer easily enough.
Built html docs (w/o style) and proofread all my changes.