Undocumented config feature not in the docs: overwrite_null

[RollingStar]

See here for info on overwrite_null:

https://github.com/beetbox/beets/pull/4987

This is a good first issue / bitesize! Even something as simple as a link to that issue, from the config docs page, would be helpful for future coders.

[bal-e]

Should the documentation basically just be "These config options allow overwriting blank metadata fields for albums or tracks"?

[RollingStar]

Honestly, it's beyond me. Whatever the code and past comments say.