Create a table in Power BI Desktop with empty text field

You could create a data table that has just the primary key of your data table and a comments field, and then create a relationship between the two.  From the Edit Query page, choose the "Enter Data" button on the Home ribbon.  Then name your table and the columns, something like ID and Comments.  You should have something similar to this:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUQpJLS5RcM7PzU3NK1GKjQUA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [ID = _t, Comments = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"Comments", type text}})
in
    #"Changed Type"

Then whenever you want to comment on any record in your full data table, come to this new table in the query editor, click on the little gear to the right of the "Source" in Applied Steps, and add the row or change the comments on existing rows.  Then when you apply changes, the new comments will be available to you to use in your reports.