[clang-tools-extra] [clang-tidy] Add a fully custom message to `bugprone-unsafe-functions` (PR #162443)

[Donát Nagy via cfe-commits]

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

The content LGTM, but there is incorrect markup in the RST files.

https://github.com/llvm/llvm-project/pull/162443