Generated documentation for the SDK

[eli-darkly]

Although the SDK contains documentation comments, they haven't been viewable anywhere except in IDEs. We are planning to put generated docs online. If you'd like to watch this issue, we will update it when the docs go online.

We've also done some editing to improve the completeness and correctness of these comments.

[eli-darkly]

After a lengthy delay, we finally have generated API documentation for the PHP SDK. It is at http://launchdarkly.github.io/php-server-sdk/ and will be updated for each release.

We're currently using a prerelease build of phpDocumentor 3, so the look of the docs may evolve further once there is a stable version of phpDocumentor.