Index: src/wp-includes/class-http.php
===================================================================
--- src/wp-includes/class-http.php	(revision 34495)
+++ src/wp-includes/class-http.php	(working copy)
@@ -159,16 +159,22 @@
 			$r['_redirection'] = $r['redirection'];
 
 		/**
-		 * Filter whether to preempt an HTTP request's return.
+		 * Filter whether to preempt an HTTP request's return value.
 		 *
-		 * Returning a truthy value to the filter will short-circuit
-		 * the HTTP request and return early with that value.
+		 * Returning a non-false value from the filter will short-circuit the HTTP request and return early with
+		 * that value. A filter should return either:
+		 *
+		 *  - An array containing 'headers', 'body', 'response', 'cookies', and 'filename' elements
+		 *  - A `WP_Error` instance
+		 *  - boolean false (to avoid short-circuiting the response)
+		 *
+		 * Returning any other value may result in unexpected behaviour.
 		 *
 		 * @since 2.9.0
 		 *
-		 * @param bool   $preempt Whether to preempt an HTTP request return. Default false.
-		 * @param array  $r       HTTP request arguments.
-		 * @param string $url     The request URL.
+		 * @param bool|array|WP_Error $preempt Whether to preempt an HTTP request's return value. Default false.
+		 * @param array               $r       HTTP request arguments.
+		 * @param string              $url     The request URL.
 		 */
 		$pre = apply_filters( 'pre_http_request', false, $r, $url );
 		if ( false !== $pre )