[clang] [docs] Fix markdown migration defects in the user manual (PR #210785)

[Reid Kleckner via cfe-commits]

rnk wrote:

Thanks for reviewing!

> Also note that a new section was added to this document yesterday that's missing in this PR.
> 
> #196392
> 
> I'm surprised that isn't flagged as a merge conflict.

I think this will appear as a conflict against the rename PR in this PR stack, so I'll have to resolve that.

https://github.com/llvm/llvm-project/pull/210785