document '_detected_errors' pattern in plugin configuration guide.

It took me a little while to figure out how to validate the keys for https://github.com/mitchellh/vagrant-rackspace/pull/43.  It turned out there is a commonly used pattern that solves this, but it isn't mentioned in the validation guide.

This PR documents the pattern in the guide, and also clarifies that the returned hash may have section keys with no values (this is the behavior of many core plugins).