drm/doc: Hide legacy horrors better

By consolidating them all into one section at the very end. And to
make double-sure that no one gets confused start with a stern warning
against any use of them. And prefix all subsections with "Legacy".

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>