[specification] `@docImport` is not specified

Dart documentation comment [specification](https://github.com/dart-lang/dartdoc/blob/main/doc/dart_documentation_comment_specification.md) mentions `@docImport` directive but this directive is not specified nor in spec nor in [directives](https://github.com/dart-lang/dartdoc/blob/main/doc/directives.md).

Please add it to the specification or to directives.