The rendered documentation is hosted at https://www.flowdock.com/api
The documentation is rendered based on MarkDown files you can find under docs.
You're free to peruse the commit history to track the changes in the Flowdock API & its docs, but a more condensed history of changes can be found at https://flowdock.com/api/changes.
Create an issue in this repository if you bump into a problem while using the API. Feel free to send a pull request if you find any errors, typos or the like.