media: debugfs-cec-error-inj: document CEC error inj debugfs ABI

Document the core of the debugfs CEC error injection ABI.

The driver specific commands are documented elsewhere and
this file points to that documentation.

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