how to add a description to local file albums?

The description field isn't presented in Song Info/Properties, but if you use the Songs layout and enable the Description column you can tap twice (slower than a double-click) in the field to edit it. This is a track level property rather than one that applies to the whole album.

tt2

Quite. You can add a description to a song. You can add the same description to all songs from the same album should you wish. The only place it will appear is in the Songs view if the Description column is enabled.

tt2