drm/vc4: kms: Document the muxing corner cases

We've had a number of muxing corner-cases with specific ways to reproduce
them, so let's document them to make sure they aren't lost and introduce
regressions later on.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Reviewed-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105135656.383350-6-maxime@cerno.tech