Title: #295: MAN: Document that the secrets provider can only be specified in a per-client
I think it could be useful to add something like "The secrets responder is configured
with a global [secrets] section and an optional per-user [secrets/users/$uid] section in
sssd.conf" just under `CONFIGURATION OPTIONS` or in some easy to spot location.
Also I wonder if it is worth mentioning herein the provider section(instead of only at the
bottom of the man page) that the `provider = proxy` can only be used in the per-user
section, not the global [secrets] section.
Otherwise looks okay from my side.
See the full comment at https://github.com/SSSD/sssd/pull/295#issuecomment-318368728