docs(api-server): update data access documentation

- Clarify read-only models and their management process
- Enhance description of special filter for country model
- Improve wording and structure for better understanding

Author: fulleni

src/content/docs/api-server/reference/data-access.mdx

@@ -32,7 +32,7 @@ The following model names are supported:
 -   `dashboard_summary`
 
 <Aside type="caution" title="Read-Only Models">
-The `country` and `language` models are configured to be **read-only** through the API (i.e., they do not support `POST`, `PUT`, or `DELETE` requests). This is an intentional design choice, as this foundational data is managed via the database seeding process.
+The `country` and `language` models are configured to be **read-only** through the API (i.e., they do not support `POST`, `PUT`, or `DELETE` requests). This is an intentional design choice. This foundational data is managed via the database seeding process.
 </Aside>
 
 <Aside type="caution">
@@ -51,10 +51,12 @@ Retrieves a paginated list of items for a specific model.
 #### Query Parameters
 
 -   **`filter`** (string, optional): A URL-encoded JSON string representing a MongoDB-style query.
-    <Aside type="note" title="Special Filter for Country Model">
-    For the `country` model, the `filter` parameter supports a special `usage` key. This allows you to retrieve countries based on their role in other data:
+    <Aside type="note" title="Special Filter for Country Model: 'usage'">
+    For the `country` model, the `filter` parameter supports a special `usage` key. This allows you to retrieve a curated list of countries that are relevant to existing content, rather than all countries globally. This is particularly useful for populating dynamic dropdowns or filters in client applications.
+
     -   `filter={"usage":"eventCountry"}`: Returns countries referenced as `eventCountry` in headlines.
     -   `filter={"usage":"headquarters"}`: Returns countries referenced as `headquarters` in sources.
+
     When using the `usage` filter for the `country` model, the response will **not** be paginated and will return the complete filtered set of countries.
     </Aside>
 -   **`sort`** (string, optional): A comma-separated list of fields to sort by (e.g., `createdAt:desc,title:asc`).