[clang] [clang][analyzer] Improve test and documentation in cstring NotNullTerminated checker (PR #112019)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

LGTM, this clarifiction is useful. I feel that these tests are very slightly too verbose, but if you already wrote them, then we might as well keep them.

https://github.com/llvm/llvm-project/pull/112019