Support for "conf.d" configuration directories and RABBITMQ_CONFIG_FILES are not documented

[lukebakken]

It needs to be as it relates to these issues, PRs and discussions:

• Original PR for the feature - Load additional Cuttlefish configuration files rabbitmq-server#2258
• Discussions and other issues:
  • rabbitmq management login failed rabbitmq-server#4319
  • Do not use RABBITMQ_LOGS to configure logging docker-library/rabbitmq#554
  • Use conf.d to enable console logging in RMQ 3.9+ docker-library/rabbitmq#555
  • Documentation seems wrong, and error message is unhelpful docker-library/rabbitmq#506 (many off-put users here)

[michaelklishin]

Happy to do it if you'd rather not spend time on docs @lukebakken :)