[clang-tools-extra] [clang-doc] fix flaky test in clang-doc (PR #101387)

[Paul Kirth via cfe-commits]

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

LGTM, but test locally w/ LLVM_ENABLE_EXPENSIVE_CHECKS=On https://llvm.org/docs/CMake.html. If that passes, then go ahead and land this, since I think the nondeterminism in output order will be accounted for.

https://github.com/llvm/llvm-project/pull/101387