media: cec-pin-error-inj.rst: document CEC Pin Error Injection

The CEC Pin framework adds support for Error Injection.

Document all the error injections commands and how to use it.

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