Documentation update: explain common error

[merriam]

In Introduction/Responses:

Change: The value of these keys will generally be a JSON object for a request on a single object and an array of objects for a request on a collection of objects.

To: The value of these keys will generally be a JSON object for a request on a single object and an array of objects for a request on a collection of objects. Malformed requests return a JSON object with a message field of "The resource you were accessing cannot be found."