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

[Jonas Toth via cfe-commits]

JonasToth wrote:

@PiotrZSL I did assign you as reviewer because I noticed you do many reviews. Please remove yourself if that was not appropriate.
I did not contribute to clang-tidy for a longer period and not sure who to assign (if any!) for the PR. It was not meant offensively or so :)

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