[linter] Update unintended_html_in_doc_comment to handle multiline doc comments

Fixes https://github.com/dart-lang/sdk/issues/64184

Change-Id: If4ad429c55555aecab275930c61409bdba11e1c8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/544200
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>