Add documentation on viewing deleted files in the diff viewer.
Review Request #9351 — Created Nov. 6, 2017 and submitted
This adds a small section on how to view deleted files in the diff
viewer, showing how a deleted file normally looks and what happens when
you click "Show content." It also talks about how these files can be
reviewed like any other.
Built the docs and looked through them.
Checked for build and spelling errors.