Abilities API: Align the content query docs with the sibling abilities

- Explain in the class docblock why the content ability lives in a
  dedicated class instead of a closure in wp_register_core_abilities(),
  as the users and settings classes do.
- Say that an empty `fields` list, like an omitted one, returns the lean
  set of common read fields.

Author: jorgefilipecosta

src/wp-includes/abilities/class-wp-abilities-content.php

@@ -18,9 +18,12 @@
  * by post type, status, author, parent, or included IDs. Raw fields are only returned for
  * posts the current user can edit.
  *
- * The class is intentionally structured around shared building blocks (exposed post type
- * discovery, schema generation, per-post formatting and permission checks) so future
- * write-oriented content abilities can reuse them.
+ * Unlike the other core abilities, which are self-contained closures registered directly
+ * in wp_register_core_abilities(), the content ability lives in a dedicated class because
+ * its callbacks and schemas share helpers: the permission and execute callbacks resolve and
+ * authorize the requested post through the same code, and the input schema, output schema,
+ * and field projection are built from the same field definitions. Future write-oriented
+ * content abilities can reuse them as well.
  *
  * This class is part of WordPress' internal implementation of the core abilities and is
  * not part of the public API. It may be changed or removed at any time without notice.
@@ -968,7 +971,7 @@ private function get_fields_input_schema(): array {
 				'type' => 'string',
 				'enum' => array_keys( $this->get_post_properties() ),
 			),
-			'description' => __( 'Limit each returned post to these fields. The `id` is always included. If omitted, a lean set of common read fields is returned. Explicit raw field requests require edit access.' ),
+			'description' => __( 'Limit each returned post to these fields. The `id` is always included. If omitted or empty, a lean set of common read fields is returned. Explicit raw field requests require edit access.' ),
 		);
 	}