Update the docs for `unintended_html_in_doc_comment`

[devoncarew]

We'd like to add unintended_html_in_doc_comment to the package:lints lint set, in the core set of lints. Before we do that we'd need to graduate the lint out of experimental and would like some documentation updates (as per dart-lang/lints#192 (comment)).

@kallentu, I see you authored this lint. Are you the best person to update the lint? @pq?

[devoncarew]

Updated the above to reflect that we don't need to toggle the experimental state here.

[pq]

@kallentu: let me know if you don't have cycles/interest and I'm happy to pitch in.

[kallentu]

Docs should be updated now. Closing.

[devoncarew]

Thanks!