Missing AIRFLOW__{SECTION}__{OPTION}__SECRET in environment variable reference

[mik-laj]

Hello,

One env variiable - AIRFLOW__{SECTION}__{OPTION}__SECRET has not been added to our environment variables reference. For moe info, see: See: http://apache-airflow-docs.s3-website.eu-central-1.amazonaws.com/docs/apache-airflow/latest/howto/set-config.html

Related:
#12773
#12772

[mik-laj]

CC: @kaxil