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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

One question I have is, towards the top of the file we convert:
```
```{eval-rst}
.. option:: -fdiagnostics-print-source-range-info
```
into
```
:::{option} -fdiagnostics-print-source-range-info
```
but towards the bottom of the file we convert:
```
```{eval-rst}
.. option:: -mno-tocdata
```
into
```
:::{option} -mno-tocdata
:no-index:
```
Why do some get `:no-index:` while others do not?

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