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

[Reid Kleckner via cfe-commits]

rnk wrote:

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

This is true, but a local `git/jj diff -w` actually does a much better job, because the lines with no change other than a de-dent serve as anchors for the rest of the diff algorithm. I find 

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

@andykaylor , can you help?

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