[clang] [NFC][analyzer] Rewrite comment header of MallocChecker (PR #165443)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

**Reviewing this PR is low priority:** it only modifies a comment and I'm reasonably confident that the new text is more accurate than the old one, so I'm planning to merge this within a few days if nobody objects. (But feel free to offer suggestions or clarifications if you have time to look at this.)

https://github.com/llvm/llvm-project/pull/165443