Elaborate on when `cred_store = ccache:...` should be used

Current docs aren't very clear about when to use this. As a result, reasonable users often make the assumption that this should be something like `cred_store = ccache:/tmp/krb5cc_%u` or `cred_store = ccache:KEYRING:%u` or what have you.

We should make explicit what this is actually intended for and that it shouldn't be otherwise used.