[AIRFLOW-2710] Clarify fernet key value in documentation
[AIRFLOW-2710] Clarify fernet key value in documentation

Closes #3574 from padwasabimasala/AIRFLOW-2710

(cherry picked from commit 6b7645261b5df69b01936838f38f7f701a667518)
Signed-off-by: Bolke de Bruin <17f52a69a3ba26c79c8a88ef8c1f26312d9cbf0e@xs4all.nl>

owlabs/incubator-airflow,CloverHealth/airflow,owlabs/incubator-airflow,CloverHealth/airflow,CloverHealth/airflow,owlabs/incubator-airflow,CloverHealth/airflow,owlabs/incubator-airflow