[clang] [analyzer][docs] Update the release notes for llvm-18 (PR #76446)

[via cfe-commits]

https://github.com/DonatNagyE commented:

There are several changes that are only relevant for the users who use alpha checkers. Perhaps it would be a good idea to put those into a separate list (or to the end of the list?).

Either way, the update looks good to me, thanks for composing it!

https://github.com/llvm/llvm-project/pull/76446