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: cbb4d3e651 ("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

Documentation/doc-guide/kernel-doc.rst

@@ -341,6 +341,51 @@ Typedefs with function prototypes can also be documented::
    */
    typedef void (*type_name)(struct v4l2_ctrl *arg1, void *arg2);
 
+Object-like macro documentation
+-------------------------------
+
+Object-like macros are distinct from function-like macros. They are
+differentiated by whether the macro name is immediately followed by a
+left parenthesis ('(') for function-like macros or not followed by one
+for object-like macros.
+
+Function-like macros are handled like functions by ``scripts/kernel-doc``.
+They may have a parameter list. Object-like macros have do not have a
+parameter list.
+
+The general format of an object-like macro kernel-doc comment is::
+
+  /**
+   * define object_name - Brief description.
+   *
+   * Description of the object.
+   */
+
+Example::
+
+  /**
+   * define MAX_ERRNO - maximum errno value that is supported
+   *
+   * Kernel pointers have redundant information, so we can use a
+   * scheme where we can return either an error code or a normal
+   * pointer with the same return value.
+   */
+  #define MAX_ERRNO	4095
+
+Example::
+
+  /**
+   * define DRM_GEM_VRAM_PLANE_HELPER_FUNCS - \
+   *	Initializes struct drm_plane_helper_funcs for VRAM handling
+   *
+   * This macro initializes struct drm_plane_helper_funcs to use the
+   * respective helper functions.
+   */
+  #define DRM_GEM_VRAM_PLANE_HELPER_FUNCS \
+	.prepare_fb = drm_gem_vram_plane_helper_prepare_fb, \
+	.cleanup_fb = drm_gem_vram_plane_helper_cleanup_fb
+
+
 Highlights and cross-references
 -------------------------------