fix: clarify filter fields in snake_case

Resolves https://github.com/go-vikunja/vikunja/issues/365

src/content/docs/usage/filters.mdoc

@@ -31,6 +31,10 @@ The available fields for filtering include:
 | `created`     | The time and date when the task was created.                                              |
 | `updated`     | The time and date when the task was updated.                                              |
 
+{% callout type="info" %}
+All fields are in `snake_case` when accessed through the api.
+{% /callout %}
+
 You can use date math to set relative dates with any date field. [See below](#date-math) for an explanation.
 
 All strings must be either single-word or enclosed in `"` or `'`. This extends to date values like `2024-03-11`.