[llvm] [doc] Improve wording for "What is considered a security issue?" (PR #69436)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

@ahmedbougacha, @andykaylor, @mrragava, @DimitryAndric, @emaste, @gburgessiv, @cuviper, @ojhunt, @pogo59, @smithp35, @pietroalbini, @serge-sans-paille, @GreatKeeper, @smaslov-intel: FYI

https://github.com/llvm/llvm-project/pull/69436