Specify how to have a fermata over an entire chord

[BaMikusi]

Looking at what Verovio (as implemented in Muscat) does, it seems that only a single note of a given chord should be bracketed, namely the first (i.e. the highest) one. This approach seems fine to me, but should explicitly be described in the specifications, since e.g. putting each note in brackets could also be a logical (though rather 'unplaine & uneasie') solution.