Docs: Fix phpdoc types for WP_Speculation_Rules members.

What was previously `mixed` is actually a speculation rule which can be more accurately be typed as `array<string, mixed>`. Additionally, the return value of `jsonSerialize` is fixed to indicate it returns an array of lists as opposed an array of associative arrays.

Developed in #8946

Follow-up to [59837].

Props justlevine, johnbillion, westonruter.
See #64238, #63268, #62503.


git-svn-id: https://develop.svn.wordpress.org/trunk@61280 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/class-wp-speculation-rules.php

@@ -21,7 +21,7 @@ final class WP_Speculation_Rules implements JsonSerializable {
 	 * Every `$rules` value is a map of `$id => $rule` pairs.
 	 *
 	 * @since 6.8.0
-	 * @var array<string, array<string, mixed>>
+	 * @var array<string, array<string, array<string, mixed>>>
 	 */
 	private $rules_by_mode = array();
 
@@ -230,7 +230,7 @@ public function has_rule( string $mode, string $id ): bool {
 	 *
 	 * @since 6.8.0
 	 *
-	 * @return array<string, array<string, mixed>> Speculation rules data.
+	 * @return array<string, array<int, array<string, mixed>>> Speculation rules data.
 	 */
 	#[ReturnTypeWillChange]
 	public function jsonSerialize() {