add unintended_html_in_doc_comment; rev to 5.1.0

[devoncarew]

• add unintended_html_in_doc_comment
• rev to 5.1.0
• close unintended_html_in_doc_comment - add to recommended set dart-lang/core#826

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[devoncarew]

The publishing validation failure is:

Validating package...
  Package validation found the following potential issue:
  The server may enforce additional checks.
  * Packages with an SDK constraint on a pre-release of the Dart SDK should themselves be published as a pre-release version. If this package needs Dart version 3.6.0-0, consider publishing the package as a pre-release instead.
    See https://dart.dev/tools/pub/publishing#publishing-prereleases For more information on pre-releases.

which is expected, given that the 3.6 sdk has not yet been released. We'll publish this as 5.1.0, depending on 3.6; on pub.dev, it'll appear as a 'preview' version. Once 3.6 becomes stable pub will automatically update 5.1.0 to be the latest (non-preview) version.