Clarify the spring boot convention with key levels (_)

[akeller]

Provide a brief explanation in the docs when we have environment variables where the ID is separated by an underscore or not.

See internal Slack thread - https://camunda.slack.com/archives/C05KHFHK5FY/p1710971396554299

@aabouzaid offered a really great response here based on Spring Boot convention (which should be intuitive for Spring devs) - https://camunda.slack.com/archives/C05KHFHK5FY/p1708951559040419?thread_ts=1708948990.342319&cid=C05KHFHK5FY

I'm not entirely sure where this should be placed in the docs, so this will require a little work to figure out where to place this info.