doc-guide: kernel-doc: tell about object-like macros

Since 2014 kernel-doc has supported describing object-like macros
but it is not documented anywhere. I should have required some
documentation for it when I merged the patch. :(

There are currently only 3 uses of this (all in DRM headers, in
include/drm/*.h).

Add object-like macro kernel-doc documentation now so that more may
know about it and use it.

Fixes: cbb4d3e6 ("scripts/kernel-doc: handle object-like macros")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240107012400.32587-1-rdunlap@infradead.org