[media] cec-ioc-adap-g-caps.rst: document CEC_CAP_NEEDS_HPD

Document the new CEC_CAP_NEEDS_HPD capability.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>