document how git-secret uses user's keyring and the git-secret repo keyring to secure and provide access to secrets, so users understand how the keyrings work and can fix situations like #638

[joshrabinowitz]

See #553 / and #638; See also #680; it is possible for people to operate on the git-secret keyring if needed (although it is discouraged).

Edit: See also #746 where people seem to be using private keys with git-secret

[joshrabinowitz]

I think we can close this now, we have documented the two keyrings pretty well.