drm: document drm_mode_modeinfo

This allows `struct drm_mode_modeinfo` references to be linkified.

Some descriptions are borrowed from struct drm_display_mode.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/BBtyuxgs3DvcrMtbRyb7KBEWUviGy1dtWO61eB4@cp3-web-016.plabs.ch