[clang] [analyzer] Mention possibility of underflow in array overflow errors (PR #84201)

[Balázs Kéri via cfe-commits]

balazske wrote:

The change looks correct, but it would be more accurate if 3 different index error cases would be possible, index is too small (negative), too large, or can be both too small and too large.

https://github.com/llvm/llvm-project/pull/84201