landlock: Fix landlock_add_rule(2) documentation

It is not mandatory to pass a file descriptor obtained with the O_PATH
flag.  Also, replace rule's accesses with ruleset's accesses.

Link: https://lore.kernel.org/r/20220506160820.524344-2-mic@digikod.net
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>

include/uapi/linux/landlock.h

@@ -62,8 +62,9 @@ struct landlock_path_beneath_attr {
 	 */
 	__u64 allowed_access;
 	/**
-	 * @parent_fd: File descriptor, open with ``O_PATH``, which identifies
-	 * the parent directory of a file hierarchy, or just a file.
+	 * @parent_fd: File descriptor, preferably opened with ``O_PATH``,
+	 * which identifies the parent directory of a file hierarchy, or just a
+	 * file.
 	 */
 	__s32 parent_fd;
 	/*