Update description of comparing changes

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What does this MR do?

The changes to the "compare changes" feature in #450834 (closed) require some small docs changes. I'll take this opportunity to combine two subheadings into one, fix the icon usage, and tidy up some wording. I think I can get away with no screenshot changes for now, but after I check the changes in #450833 (closed), that might change.

Related issues

Related to Update diff preferences dropdown (#450834 - closed)

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