Adding documentation to the digest_auth method

I fixed the title from wsse_auth to digest_auth, and I also made a note that you must have the httpclient gem included (or another one of the HTTPI adapters that supports HTTP Digest authentication) for it to work.

I didn't have any of the other gems, so it was relying on net_http, which doesn't support digest auth.  It wasn't complaining, it just wasn't working.  As soon as I included httpclient, it started to work.