[clang-tools-extra] [clang-doc][NFC] Use qualified auto (PR #136394)

[Paul Kirth via cfe-commits]

ilovepi wrote:

### Merge activity

* **Apr 19, 2:32 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/136394).


https://github.com/llvm/llvm-project/pull/136394