Improve docs on connecting to password-protected buckets

First comment:

People are having trouble connecting SG to a password-protected Couchbase Server bucket. See [this thread](https://groups.google.com/d/msgid/mobile-couchbase/a0d50c3a-46ce-4e65-8a0b-88388e06bdee%40googlegroups.com?utm_medium=email&utm_source=footer), in particular the message where Jeremy describes the solution (5 August 8:58pm).

It sounds like part of the problem is confusion about what the credentials are for a Couchbase Server bucket; that's not something I know anything about. Maybe we can link to some Couchbase Server docs?

see other comments: https://github.com/couchbaselabs/couchbase-mobile-portal/issues/108