Update the faye engine doc to reflect implementation.

[viktomas]

back port of https://gitlab.com/gitlab-org/gitter/faye/merge_requests/5

See https://github.com/faye/faye/blob/master/src/engines/memory.js#L71-L103 for memory engine implementation

And https://github.com/faye/faye-redis-node/blob/master/faye-redis.js#L122-L142 for the redis engine implementation

[jcoglan]

This does not look accurate to me.

First, the core faye package does not depend on this interface; it actually doesn't currently depend on the engine API supporting callbacks at all; these were added for forwards compatibility as we may need to add more detail to this interface.

Second, it doesn't describe the implementation. The memory engine returns a single argument to the callback (true) and the redis engine returns no arguments at all. Both do support passing a context argument but I don't want this to be a requirement of the interface definition.

Do you have some more context on why you made this documentation change?