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

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> What I see is just reformatting, fixing an occasional typo, deleting the "FUTURE" remarks, and telling people that even though we say nothing is security-relevant, that shouldn't stop them from filing security issues. That all seems fine.
> 
> Sorry to have missed the Dev Meeting round table (had a conflict) and the last meeting (I've been ill). We have seemed to accept security reports as valid in a couple of areas, so "none" probably isn't really correct, but we should discuss that in a future meeting before updating this description.

Thanks for the review. My intent for the change is exactly what you saw :).
Agreed, we should improve the description of which security reports we accept to be more accurate than "none".

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