Reword the error message for auth and clarify the manual page

The manual page talked about "recommended to use", but it is
mandatory.

Change-Id: I7827d6d09ad077fd4e0ebebe97eff1873b2ee573
Reviewed-on: http://review.couchbase.org/29058
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>