Accessible sticky notes and callouts?

Can anyone recommend a best practice for making accessible comments in PDFs? And but comments I mean the text annotations and free text annotations, like a sticky note or callout? The way I am reading the UA standard I would put the annotation OBJR in a semantic tag (I was thinking <Note> but maybe <Form> makes more sense?) and if the Contents key is not present add some alternative text to the annotation. Is it useful to always add alternative text, similar to the Link annotation requirement in PDF-UA or is that just going to repeat text?
Thanks!