media: media/cec-core.rst: update adap_enable doc

The description of adap_enable was out-of-date. Improve it so that it
corresponds to what really happens.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>