Document search filters like `published:"this month"`

[samanthavbarron]

It looks like search features like published:"this month" are supported, but undocumented.

I was about to open this as a feature request in the main repo but found that some of these are implemented:

https://github.com/omnivore-app/omnivore/blob/341bf20d72a5ed92d215a72660e0a2760c2831a1/packages/api/src/services/library_item.ts#L404

As far as I can tell this only applies to the published field.

[cullen-dev]

Bump

Not only do we need this documented, but please extend this functionality to the saved field as well! This is a must!