Mark @api annotation as useful

[simPod]

Some third party libs use @api annotation to generate documentation etc. Not having it in useful annotations IMO might prevent adoption of this CS by other libraries.

[alcaeus]

We've had the discussion previously when adding it to the forbidden annotations: #28 (comment). Money quote:

Any public or non-final protected is typically public API by design.

For the Doctrine projects, any method as described in the quote is considered part of the public API - we specifically mark some classes or methods as @internal if they are not intended to be used as public API - that's why we've added it to the forbidden annotations. Other projects that rely on the @api tag may always override this sniff in a custom ruleset to expand the list of forbidden/allowed annotations. 👍

[Ocramius]

Closing as this goes directly against what we define as "API". Anything public (or protected) that isn't marked as @internal, as @alcaeus just described.