drm/ttm: fix ttm_bo.h kernel-doc warnings

Some renames, some formatting fixes, add some missing documentation.

v3: Fix struct ttm_buffer_object .sg documentation (Christian)

Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240308160750.3741833-1-jani.nikula@intel.com