Add to api docs

docs/api.md

@@ -162,6 +162,17 @@ including `gt` (>), `gte` (>=), `lt` (<), `lte` (<=), and `range`.
 Lastly, document link fields support a `contains` operator that behaves
 like a "is superset of" check.
 
+Date fields also accept relative expressions supported by
+[dateparser](https://dateparser.readthedocs.io/en/stable/), such as `today`,
+`in 30 days`, or `1 month ago`. For example, dates within the next 30 days:
+
+```json
+["expiry", "range", ["today", "in 30 days"]]
+```
+
+Expressions resolve each time the query runs, using `PAPERLESS_TIME_ZONE` and
+`PAPERLESS_DATE_PARSER_LANGUAGES` (automatic language detection when unset).
+
 ### `/api/search/autocomplete/`
 
 Get auto completions for a partial search term.