drm/vc4: Improve the KUnit documentation

The command-line can be expressed using a code-block, and we were
missing which architectures were available.

Suggested-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208094727.2848310-1-maxime@cerno.tech

Documentation: gpu: vc4: Add KUnit Tests Section

Now that we have VC4-specific tests in place, let's document them
properly.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-20-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

drm/vc4: Add a paragraph at the top of vc4 docs introducing what it is.

This makes for more sensible documentation of the whole module than
jumping straight into the details of display.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227201144.10970-5-eric@anholt.net

drm/vc4: Add RST to bring together vc4 kerneldoc.

This doesn't yet produce coherent documentation of the module, but at
least gets the kerneldoc built and somewhat glued together.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227201144.10970-3-eric@anholt.net