Added manual documentation for updating and reviewing file attachments

Review Request #6801 — Created Jan. 17, 2015 and submitted — Latest diff uploaded

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Review Board
master
20f0351...

Reviewers

Added an initial png for the docs "file-revision-selector.png" that demonstrates the attachment slider for the user.
Also added docs for the image and text diffing in file attachments, features that were exposed by the attachment slider.

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