Document SECRETS_VERBOSE env var in man pages

[joshrabinowitz]

This separates the documentation of the SECRETS_VERBOSE environment variable support (PR #393) from it's implementation.

[joshrabinowitz]

May as well also use this opportunity to clarify the docs regarding all the supported SECRETS_* env vars.

[sobolevn]

@joshrabinowitz yes, that's totally required. Since we already have a bunch of them!

Thanks for this! You do an awesome work. 🚀

[joshrabinowitz]

Thank you @sobolevn ! It's a satisfying project to work on, and it's nice to collaborate with you.