Emit line numbers in `write-rustdoc-hide-lines`

As a follow up to #1101, the `write-rustdoc-hide-lines` tool could be improved by emitting which line are not formatted properly. These lines can also be [emitted in CI](https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-error-message), which will make it more clear what is causing the job to fail.

Locate the write-rustdoc-hide-lines entry point and trace how it detects lines that are not formatted properly. Check how the tool is invoked in CI, then verify that the reported line numbers appear both in its output and as GitHub Actions error annotations when the job fails.

Written by the indexing model from the issue text.