The CEC_LOG_ADDRS_FL_CDC_ONLY flag is missing at the documentation,
causing this warning:
	Documentation/output/cec.h.rst:6: WARNING: undefined label: cec-log-addrs-fl-cdc-only (if the link has no caption the label must precede a section header)

Add a documentation for it, based on the commit that introduced the
flag.

Fixes: a69a168a ("[media] cec: add proper support for CDC-Only CEC devices")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>