[clang-tools-extra] [clang-tidy][NFC] Add `doc8` documentations (PR #167269)

[Baranov Victor via cfe-commits]

================
@@ -451,6 +451,19 @@ your build tree.  Make sure that your check is correctly shown in the release no
 list of checks.  Make sure that the formatting and structure of your check's documentation
 look correct.
----------------
vbvictor wrote:

```suggestion
list of checks.  Make sure that the formatting and structure of your check's documentation
look correct: there is no trailing whitespaces and lines are no longer than 80 characters. To validate your files, please use ``doc8`` as described below.
```

https://github.com/llvm/llvm-project/pull/167269