Added doc for the captions api.
Clarified what happens when you incorrectly use the create/update methods in the captions API.
Add example response to Captions#show api doc.
More changes to the captions api as suggested by Brendan.
More improvements to the captions docs about error handling and such things.
Make the language parameter of the Captions#delete method default to eng.
Added a show action for the captions api.
Added a note about how true support for multiple languages will be added later in the captions api doc.