These have some diff functionality but are not "diff / merge tools". There are diff tools, though, targeted at lawyers for legal work: "show me the differences between these two contracts" and the such (which most developer-oriented tools actually suck at as they care too much about "lines" and whitespace).
I don't think it's that black and white? Google Docs, for example, lets you individually review and accept/deny/revise each individual suggested change. It's not a 3-way merge like programmers are used to seeing, but it's the same idea... you start with an original, see someone else's changes, and decide which to keep, or you can take their changes and further edit. And comment in-line too.
I've never used a lawyer's diff tool, but my IDE (IntelliJ) ignores whitespace and lines.
