drm/connector: add ref to drm_connector_get in iter docs

Mention that connectors need to be referenced manually if they are
to be accessed after the iteration has progressed or ended.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/KRoUI7OC9lRIvk3YzdGm6tcMVAVlG1fR78Ll7kTZZT4@cp3-web-051.plabs.ch