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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

The diff algorithm makes this pretty hard to review because text seems to have moved just enough to confuse the highlighting. :-D

The changes look reasonable in general, but another set of eyes to spot-check might be helpful.

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