[clang-tools-extra] [clang-tidy][docs] improve documentation on cppcoreguidelines-narrowing-conversions (#111510) (PR #118209)

[Piotr Zegar via cfe-commits]

https://github.com/PiotrZSL requested changes to this pull request.

- Formatting
- Please think about simplifying this.

https://github.com/llvm/llvm-project/pull/118209