scripts: kernel-doc: Clarify missing struct member description

The output "or member" should be more specific, instead saying "struct
member".

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231215001513.work.563-kees@kernel.org