Note [Api annotations]

[Ömer Sinan Ağacan]

I see some weird comments like

        --  - 'ApiAnnotation.AnnKeywordId' : 'ApiAnnotation.AnnOpen',
        --      'ApiAnnotation.AnnVbar','ApiAnnotation.AnnComma',
        --      'ApiAnnotation.AnnClose'

        -- For details on above see note [Api annotations] in ApiAnnotation

in some files, but Note [Api annotations] in compiler/parser/ApiAnnotation.hs
doesn't say anything about those comments. Can someone update the note to
explain what are those comments for?