doc: Document the new inline struct member kernel-doc style

We don't just need better doc toolchains, we also need better docs for
our doc toolchain!

v2: Make sure we don't have foo twice (Jani).

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>