Feature: enhances custom field search API

docs/api.md

@@ -235,12 +235,6 @@ results:
 Pagination works exactly the same as it does for normal requests on this
 endpoint.
 
-Certain limitations apply to full text queries:
-
-- Results are always sorted by search score. The results matching the
-  query best will show up first.
-- Only a small subset of filtering parameters are supported.
-
 Furthermore, each returned document has an additional `__search_hit__`
 attribute with various information about the search results:
 
@@ -280,6 +274,67 @@ attribute with various information about the search results:
 - `rank` is the index of the search results. The first result will
   have rank 0.
 
+### Filtering by custom fields
+
+You can filter documents by their custom field values by specifying the
+`custom_field_lookup` query parameter. Here are some recipes for common
+use cases:
+
+1. Documents with a custom field "due" (date) between Aug 1, 2024 and
+   Sept 1, 2024 (inclusive):
+
+   `?custom_field_lookup=["due", "range", ["2024-08-01", "2024-09-01"]]`
+
+2. Documents with a custom field "customer" (text) that equals "bob"
+   (case sensitive):
+
+   `?custom_field_lookup=["customer", "exact", "bob"]`
+
+3. Documents with a custom field "answered" (boolean) set to `true`:
+
+   `?custom_field_lookup=["answered", "exact", true]`
+
+4. Documents with a custom field "favorite animal" (select) set to either
+   "cat" or "dog":
+
+   `?custom_field_lookup=["favorite animal", "in", ["cat", "dog"]]`
+
+5. Documents with a custom field "address" (text) that is empty:
+
+   `?custom_field_lookup=["OR", ["address", "isnull", true], ["address", "exact", ""]]`
+
+6. Documents that don't have a field called "foo":
+
+   `?custom_field_lookup=["foo", "exists", false]`
+
+7. Documents that have document links "references" to both document 3 and 7:
+
+   `?custom_field_lookup=["references", "contains", [3, 7]]`
+
+All field types support basic operations including `exact`, `in`, `isnull`,
+and `exists`. String, URL, and monetary fields support case-insensitive
+substring matching operations including `icontains`, `istartswith`, and
+`iendswith`. Integer, float, and date fields support arithmetic comparisons
+including `gt` (>), `gte` (>=), `lt` (<), `lte` (<=), and `range`.
+Lastly, document link fields support a `contains` operator that behaves
+like a "is superset of" check.
+
+!!! warning
+
+    It is possible to do case-insensitive exact match (i.e., `iexact`) and
+    case-sensitive substring match (i.e., `contains`, `startswith`,
+    `endswith`) for string, URL, and monetary fields, but
+    [they may not work as expected on some database backends](https://docs.djangoproject.com/en/5.1/ref/databases/#substring-matching-and-case-sensitivity).
+
+    It is also possible to use regular expressions to match string, URL, and
+    monetary fields, but the syntax is database-dependent, and accepting
+    regular expressions from untrusted sources could make your instance
+    vulnerable to regular expression denial of service attacks.
+
+    For these reasons the above expressions are disabled by default.
+    If you understand the implications, you may enable them by uncommenting
+    `PAPERLESS_CUSTOM_FIELD_LOOKUP_OPT_IN` in your configuration file.
+
 ### `/api/search/autocomplete/`
 
 Get auto completions for a partial search term.