get_item is passed an array as opposed to a WP_REST_Request in several places (NOTE, line numbers may change but are correct at the time of submission):

• class-wp-rest-attachments-controller.php, line 85, 125
• class-wp-rest-comments-controller.php, line 203, 267
• class-wp-rest-posts-controller.php, line 235, 309
• class-wp-rest-terms-controller.php, line 202, 260
• class-wp-rest-users-controller.php, line 192, 258, 316

The way I see it there are 3 different possible fixes:

1. The documentation (this PR) needs to be corrected to allow for a generic array.
2. Each of these instances need to instantiate and use a WP_REST_Request
3. Refactor get_item to use a helper function to maintain type integrity though I am not sure what this would look like per se.