drm: writeback: Fix doc that says connector should be disconnected

During iteration process one of the proposed mechanism for not
breaking existing userspace was to report writeback connectors as
disconnected, however the final version used
DRM_CLIENT_CAP_WRITEBACK_CONNECTORS for that purpose.

Change-Id: I2319d099f7669094c8530f1521abdbca08e76486
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/238399/