drm/bridge: document HDMI CEC callbacks

drm/bridge: document HDMI CEC callbacks

Provide documentation for the drm_bridge callbacks related to the
DRM_BRIDGE_OP_HDMI_CEC_ADAPTER flag.

Fixes: a74288c8ded7 ("drm/display: bridge-connector: handle CEC adapters")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20250611140933.1429a1b8@canb.auug.org.au
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250801-drm-hdmi-cec-docs-v1-1-be63e6008d0e@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>