projectID requirement not consistently documented

[ptone]

on:
https://googlecloudplatform.github.io/google-cloud-node/#/docs/google-cloud/0.40.0/google-cloud

Documents on compute engine setup as:

 var gcloud = require('google-cloud')();

But runtime errors, and the sample on the package home page make clear that the projectID has to be explicit (not sure why can't pick up from metadata server?)

https://googlecloudplatform.github.io/google-cloud-node/#/

[stephenplusplus]

We can't make an async request to the metadata server, but we do pick it up from the environment variable GCLOUD_PROJECT. We mention this in our Authentication guide, but it would be helpful to clear this up in our config parameter docs.

Sent a PR, please take a look: #1601