CCBC-179 Provide an example about least privilege principle

An example and clear statement that for usual data connection user
should specify bucket credentials instead of admin.

Change-Id: I6a4940bd3e02f49a18b0dc91498951e991ab0be3
Reviewed-on: http://review.couchbase.org/26108
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>