`EngineDiscoveryListener` Javadoc contains incorrect description

[sbrannen]

The class-level Javadoc for EngineDiscoveryListener was copied from EngineDiscoveryRequest and only partially edited, resulting in the following.

/**
 * {@code EngineDiscoveryListener} contains {@link TestEngine} access to the
 * information necessary to discover tests and containers.

We should fix that.