Fix `single_match` lint being emitted when it should not (#13765)

We realized when running `clippy --fix` on rustdoc (PR comment
[here](https://github.com/rust-lang/rust/pull/133537/files#r1861377721))
that some comments were removed, which is problematic. This PR checks
that comments outside of `match` arms are taken into account before
emitting the lint.

changelog: Fix `single_match` lint being emitted when it should not