Title: #390: NSS: Add option to disable memcache
@fidencio I mentioned that I think we should add warning to the man page, but I was not
sure if we end up deleting the man page, so I did not add the change immediately.
Looking at the discussion now I think more people agree that we should document it, so I
added one more patch with the warning. The previous patch is the same. Please check if the
wording in the new patch is appropriate.
See the full comment at https://github.com/SSSD/sssd/pull/390#issuecomment-331852770