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

[Balazs Benics via cfe-commits]

steakhal wrote:

I've decided to fix the checker docs for the `security.insecureAPI.DeprecatedOrUnsafeBufferHandling`.
It turned out to be an easy one as the only missing function was `fprintf`, so it was already well in sync. I bundled that single word change with this PR for simplicity.

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