The context of this question is choosing tools for writing design specifications for software projects.
These documents will be written and maintained by architects and developers, I'm not talking about marketing requirements. Some of them may be shared outside the team, but only in a processed, non-editable form (PDF, all the docs are assumed being able to be exported to this format).
These are architecture documents, describing the structure of code components, implementation methods, protocols, data formats, etc. They take the form of text with diagrams and identifier names and the occasional code snippets, this isn't about API documentation that might be generated from source files.
The docs will be under source control, fortunately nobody here needs to be educated about that. It's inevitable that versioning will arise over time: we do maintain old versions of the software. Issue tracking might not be adhered to as strictly for documentation as for code.
How important is it to be able to easily compare and merge such documents? We have diverging opinions in the team, ranging from “nobody ever merges documentation and if needed Word has a merge tool” to “merging is crucial and
git merge must work”. Note “Word has a merge tool” is something that I quote but don't agree with, having had the painful experience of merging two bugfixes that I had made to two copies of the same document
I have a vague memory of a rule in some company (Google, perhaps, since it's so often cited) that “if you can't merge it, it doesn't exist”, but I'm unable to find it now.
I'm looking for either well-reasoned arguments, or authoritative-looking (and preferably motivated) citations.