DOC: ssl: Extra files loading now works for backends too

DOC: ssl: Extra files loading now works for backends too

When implementing the server side certificate hot update, the ckch
mechanism was used on the backend side in order to mimic the frontend
certificate management and to enable server line certificate update via
the CLI (see GitHub issue #427). As an unexpected side effect, we now
also look for ssl extra files (cert.pem.key, cert.pem.ocsp ...) for the
backend side.
This patch updates the documentation accordingly.

This answers to GitHub issue #845.