drm: document drm_mode_get_connector

Document how to perform a GETCONNECTOR ioctl. Document the various
struct fields. Also document how to perform a forced probe, and when
should user-space do it.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4NxrTtynzPiPX4SOCzxmA1sRB8fVLfeiabVpi5j3Y@cp7-web-041.plabs.ch