I'd like to bring up again this feature request in the original Allusion issues, which is adding a simple notes text field that supports line breaks. There we can write any notes or descriptions about an image. Making the text in this field searchable would be really helpful so we can quickly find a specific image by searching for keywords written in the notes field.

If you are interested in maybe implementing a more detailed system, to reference another tag management program, TagStudio has the option to define multiple text fields for an image such as author, artist, URL, description, notes, source. I like this system but I prefer Allusion's UI.