Update some text and examples in the spec.
Review Request #11735 — Created July 20, 2021 and submitted
This cleans up some of the language on the introductory page for DiffX,
and provides additional examples on the "Problems with Diffs" page for
Subversion's handling of deleted files.
Built the docs and read through the new language.
Viewed the rendered diff examples and ensured they looked correct.